J G COLLEGE OF NURSING

AHMEDABADSUBJECT: ADVANCE NURSING

PRACTICE

TOPIC: ASSIGNMENT ON EPIDEMIOLOGY

SUBMIITED TO

PROF. YONATAN SIR

J G COLLEGE OF NURSING

AHMEDABAD SUBMITTED BY

BINAL D JOSHI

FY M SCC NURSING

J G COLLEGE OF NSG

AHMEDABAD

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EPIDEMIOLOGY

• Epidemiology is the study of patterns of health and illness and associated factors at the population level. It is the cornerstone method of public health research, and helps inform evidence-based medicine for identifying risk factors for disease and determining optimal treatment approaches to clinical practice and for preventative medicine. In the study of communicable and non-communicable

• Epidemiology is the study of the distribution and determinants of health-related states or events (including disease), and the application of this study to the control of diseases and other health problems. Various methods can be used to carry out epidemiological investigations: surveillance and descriptive studies can be used to study distribution; analytical studies are used to study determinants.

Introduction to Epidemiology

Objectives:

Define epidemiology.

Understand, recognize, and explain the scientific principles behind population-based observational studies.

Demonstrate familiarity with causation models.

Key terms:

Epidemiology defined

Scientific Premise of

Epidemiology

Epidemiologic Approach

Causation Models of Disease

Chain of Infection

ReservoirVehicle

(Transmission)Host

Epidemiological Triad

Agent

EnvironmentWheel of Causation

Necessary SufficientWeb of

Causation

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Introduction to Epidemiology

Epidemiology defined: “the study of the distribution and determinates of health related states in specified populations and the application of this study to control health problems.”

Predetermined or chance occurrence? – Determinism verses Stochastic models.

Underlying premise of epidemiology: “disease, illness, and ill health are not randomly distributed in a population.”

• Determinism: • Occurrences are the result of natural law which allow the

development of models to predict occurrences. • Deterministic models imply exact prediction if all underlying factors

and laws are known. • Probability (stochastic) models:

• Occurrences occur within a range of probabilities and predictions are based on the probability of the occurrence happening.

• Stochastic models use presumed underlying distributions to model the likelihood of an occurrence.

The complexity of nature precludes the knowing of all relevant factors except in the most simple of systems; therefore we must use probability models based on the best current available information. This means that absolute prediction is not possible, but we may know the likelihood of an event happening.

Specific objectives of epidemiology:

• “to identify the etiology or cause of a disease and the risk factors.” • “to study the natural history and prognosis of disease”. • “to evaluate new models of health care delivery.” • “to provide a scientific foundation for regulatory decisions relating to

health or environmental problems.”

Other uses of epidemiology:

• “to provide a clue to changes taking place over time…” • “to identify subgroups in the population who are at high risk for

disease.” • “to determine the best or most appropriate types of primary and

secondary prevention.’ • Primary prevention prevents disease in the well individual. Example:

vaccination.

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• Secondary prevention limits disease by early detention usually through screening programs.

Epidemiology and Clinical Practice: Diagnosis, Therapy, and Prognosis are all based on the use of epidemiological data about the disease and the individual’s risk factors.

Epidemiological Approach: A search for associations between risk factors and disease; then to derive appropriate inference about causal relationships.

Historical Examples:

• Daniel- biblical nutritional clinical trial. • Hipprocates- risk factors “earth, air, water”. • Jenner- observations on smallpox to vaccination. • Farr- relationship of altitude to cholera. • Snow- relationship of water supply to cholera.

Causation Models

Chain of Infection.

• Reservoir (of infectious agents) - Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host.

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• Vehicle (Transmission)- any mechanism by which a susceptible host is exposed to an infectious agent.

• Host- any susceptible person.

Types of disease transmission

• Direct:

Direct and immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact as by touching, biting, kissing or sexual intercourse, or by direct droplet spray onto the mucous or conjunctiva membranes of the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking (usually at a distance of 1 meter or less).

• Indirect:

• Vehicle-borne –

Contaminated inanimate materials or objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings; water, food, milk, biological products including blood, serum, plasma, tissues or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry.

The agent may or may not have multiplied or developed in or on the vehicle before being transmitted.

• Vector-borne:

• Mechanical: Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.

• Biological: Propagation (multiplication), cyclic development, or a combination of this (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to humans.

• An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission); transstadial transmission indicates its passage from one stage of life

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cycle to another, as nymph to adult. Transmission may be by injection of salivary gland fluid during biting or by regurgitation or deposition on the skin of feces or other material capable of penetrating through the bite wound or through an area of trauma from scratching or rubbing.

• This transmission is by an infected nonvertebrate host and not simple mechanical carriage by a vector as a vehicle. However, an arthropod in either role is termed a vector.

• Airborne - The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of particles in the air consisting partially or wholly of microorganisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence.

• Particles in the 1-to 5-um range are easily drawn into the alveoli of the lungs and may be retained there. Not considered as airborne are droplets and other large particles that promptly settle out (see Direct transmission, above).

• Droplet nuclei: Usually the small residues that result from evaporation of fluid from droplets emitted by an infected host (see above). They may also be created purposely by a variety of atomizing devices, or accidentally as in microbiology laboratories or in abattoirs, rendering plants or autopsy rooms. They usually remain suspended in the air for long periods of time.

• Dust: The small particles of widely varying size that may arise from soil

(as, for example, fungus spores separated from dry soil by wind or mechanical agitation), clothes, bedding or contaminated floors.

Classical disease model for infectious diseases

• Consider tuberculosis applied to the chain of infection. The reservoir is an infected individual who is shedding the bacterium in his sputum or from expelled fluids. Transmission is by cough or other contact with expelled fluids. Host is a susceptible individual.

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• Epidemiological Triad (Triangle).

• Emphasis on relationship of disease to the host, the agent and environmental factors.

• Model works with both infectious or non-infectious diseases. • For TB: In this model the agent is the TB bacterium; host factors

include non-immune, weakened resistance, poor nutrition; and environmental factors include crowded conditions, poor ventilation, and bad sanitation.

Web of Causation: “Web” of interconnected factors which lead to disease.

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• Works with complex disease etiologies. • Developed for use with diseases with many cofactors. • Example above is Lung Cancer.

TB in this model: The factors of exposure to the agent, poor nutrition, crowding, poverty, low immunity, and concurrent disease may interact directly or indirectly to cause disease.

Wheels of Causation.

• Disease factors in singular or plural (necessary and/or sufficient) are required for disease occurrence.

• If “A” is “necessary”, then disease will not occur without it, but “A” may require other factors to be sufficient (example: TB). Several factors together may be sufficient for disease but no one factor may be necessary in itself (example: CHD); several sufficient wheels may exist for a disease.

• Wheel of causation illustrates how disease may occur through the interplay of several causal factors without any being necessary in itself.

TB again: One sufficient wheel might include: contact with a carrier, low immunity, and crowding; another wheel might include contact with a carrier, having AIDS, poor hygiene. Both wheels are sufficient to produce disease, but factors differ. But for TB, both wheels must have the TB organism, is a necessary factor for TB to occur.

Additional terms to understand:

• Clinical disease. • Nonclinical disease. • Preclinical disease. • Subclinical disease. • Chronic disease. • Latent disease. • Carrier. • Endemic, Epidemic, Pandemic. • Herd Immunity.

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• Incubation period. • Attack rate.

Who? Where? When?

• The Person, Place, & Time indicators of disease occurrence. • To identify the determinants and distribution of disease occurrence.

Person, place, and time factors are in fact the determinants we wish to understand. Are they associated with an outcome? Then is there evidence of a causal association? If the evidence supports a causal association, then a factor is said to be a risk factor for that outcome. From our evidence, we can often quantify the increased risk and determine (at least from a population standpoint) the risk reduction that can be achieved if that risk factor is removed.

EPIDEMIOLOGICAL APPROACH

The Epidemiological Approach to problems of health and disease is based on two major foundations.

a) asking questionsb) making comparison

a. Asking questions:

Epidemiology has been defined as “a means of learning or asking questions…..and getting answers that lead to further questions”. For example, the following questions could be asked.

RELATED TO HEALTH EVENTS:a. What is the event?b. What is its magnitude?c. Where did it happen?d. When did it happen?e. Who are affected?f. Why did it happen?

RELATED TO HEALTH ACTION:a. What can be done to reduce this problem and its consequences?b. How can it be prevented in the future?c. What action should be taken by the community by the health

services? By other sectors? Where and for whom these activities be carried out?

d. What resources are required? How are the activities to be organized?

e. What difficulties may arise, and how might they be overcome?Answer to the above questions may provide clues to disease actiology, and

help the epidemiologist to guide planning and evaluation.

b. Making comparisons

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The basic approach in epidemiology is to make comparisons an draw inferences. This may be comparison of two (or more groups). One group having the disease (or exposed to risk factor and the other group (s) not having the disease (or not exposed to risk factor), or comparison between individuals. By making comparisons, the epidemiologist tries to find out the crucial differences in the host and environmental factors between those affected and not affected. In short the epidemiologist weighs, balances and contrasts, clues to actiology come from such comparisons.

One of the first considerations before making comparisons is to ensure what is known as “comparability” between the study and control groups. In other words, both the groups should be similar so that “like can be compared with like”.

For facts to be comparable they must be accurate and they must be gathered in a uniform way.

Ex. The study and control groups should be similar with regard to their age and sex composition, and similar other pertinent variables.

The best method of ensuring comparability, in such cases, is by randomization or random allocation.

Where random allocation is not possible (as in case control and cohort studies) what is known as “matching” is done for selected characteristics that might can found the interpretation of results.

Another alternative is standardization which usually has a limited application to a few characteristics such as age, sex and purity. These biostatistical concepts are elaborated in the following pages. It may be mentioned that international comparisons may be difficult because of difference in terminology. It requires standardization of definitions, classifications, criteria and nomenclature.

METHODS OF EPIDEMIOLOGY

The primary concern of the epidemiologist is to study disease occurrence in people, who during the course of their lives are exposed to numerous factors and circumstances, some of which may have a role in disease actiology. Unlike the clinician or the more precisely, the epidemiologist employs carefully designed research strategies to explore disease actiology.

Epidemiological studies can be classified as observational studies and experimental studies with further subdivisions.

1. Observations studies2. Description studies3. Analytical studies

i. Ecological or correlational with populations as unit of studyii. Cross-sectional or prevalence, with individual as unit of studyiii. Case-control or case-reference, with individuals as unit of studyiv. Cohort or follow-up with individuals as unit of study.

2. Experimental studies intervention studies

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a. Randomized clinical with patients as Controlled trails trails unit of study.

b. Field trails Community with health people as Interventionunit of study with communities Studies as unit of study

c. Community trailsThese studies or methods cannot be regarded or water tight compartments.

They complement one another.

In all epidemiological studies, it is essential to have a clear definition of a case of the disease being investigated and of an exposed person. In absence of clear definitions of disease and exposure, great difficulties are likely to be experienced in interpreting the data.

Descriptive EpidemiologyIt is concerned with the distribution of disease or any health related event in

terms of person, place and time. Hypothesis attempts to answer question like; which population does or does not develop a disease or a health related event in what geographic locations it is more or least common and how the frequency of occurrence varies over time. Descriptive studies are usually the first phase of an epidemiological

Procedures in Descriptive studies1. Defining the population to be studied2. Defining the disease under study3. Describing the disease by

a. timeb. placec. person

4. Measurement of disease5. Comparing with known indices6. Formulation of an actiological hypothesis

1. Defining the populationTo define the “population base” not only in terms of the total number, but

also composition in terms of age, sex, occupation, cultural characters and similar information needed for the study. The “defined population” can be the whole population on a geographic area, or more often a representative sample taken form it. The defined population needs to be large enough so that age, sex and other specific rates are meaningful. The community chosen should be stable without migration into or out of the area. It should be clears who does and who does not belong to the population, as far example, visitors and relations, perhaps the most essential ingredients is community participation which must be forthcoming. Further more, the population should not be overtly different from other communities in the region. Finally, a health facility should be close enough to provide relatively easy access for patient’s requiring medical services. In the famous Framingham Heart study in US, all

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the above criteria were taken into consideration in choosing the study population.

2. Defining the disease under studyIn this the main concern of the epidemiologist is to obtain an accurate

estimate of disease in a populations needs definitions that is both precise and valid to enable him to identify those who have the disease form those who do not. That is looks out an “operational definition” i.e. a defined population with a degree of occuracy.

For eg. Tonsillitis clinically defined as an inflammation of the tonsils caused by infection usually with street to coccus pyogenes.

But the operational definition of tonsillitis is the presence of enlarged, red tonsils with while excludes which on throat swab culture grow predominantly S. pyogenes.

3. Describing the DiseaseIt includes the time, person and the place and identifying those

characteristics associated with presence or absence of disease in individuals. This involves systemic collection and analysis of dates. Epidemiologists have identified three kinds of time trends or fluctuations in disease occurrence

1. short term fluctuations2. periodic fluctuations3. long term or secular trends

1. Short term fluctuations:In short term fluctuations has three types of epidemics. Those are,

a. Common – source epidemicsi. Single exposure or point source epidemicsii. Continuous or multiple exposure epidemics

b. Propagated epidemic:i. Person to personii. Arthropod vectoriii. Animal reservoir

c. Slow (modern epidemics)

a. Common source epidemics:

1. Common source, single exposure epidemics:The main features of a point source epidemic are

The epidemic curve rises and falls rapidly, with no secondary waves. The epidemic tends to be explosive, there is clustering of cases within a

narrow interval of time and All the cases develop within one incubation period of disease.

They can result form intimation of the environment by industrial chemicals or populations.

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Eg, Bhopal gas tragedy in India and mine Mata disease in Japan resulting from consumption of fish containing high concentration of methyl mercury.

ii. Continuous or multiple exposure

Sometimes the exposure form the same source prolonged – continuous, repeated or intermittent – not necessarily at the same time or place.

For eg. Water borne cholera, the epidemic reaches a sharp peak, but tails off gradually over a longer period of time.

b. Propagated epidemics:A propagated epidemic is most often of infectious origin and results from

person to person transmission of an infectious agent.

2. Periodic fluctuations:In their we can see seas and trend and cyclic trend

a. Seasonal trendSeasonal variation is a well known characteristic of many communicable

diseases.

Eg. Measles, voicella, gastro intestinal infections are prominent in summer months because of warm weather and rapid multiplication of files.

b. Cyclic trend:The term “cyclic trend” implies changes in the occurrence of disease over a

short period of time, generally several days, weeks, months etc.

For eg. Measles in the pre-vaccination are appeared in cycles with major peaks every 2-3 years and rubella every 6-9 years.

3. Long term or secular trends:The term secular trend implies changes in the occurrence of disease over a

long period of time, generally several years of decades.

For eg. Coronary heart disease, lung cancer and diabetes which have shown a consistent upward trend in the developed countries during the past 50 years or so, followed by a decline of such diseases as tuberculosis, typhoid fever, diphtheria and polio.

4. Measurement of disease:It is mandatory to have a clear picture of the amount of disease “(disease

load)” in the population.

This information should be available in terms of mortality, morbidity, disability and soon and should preferably morbidity has two aspects. Measurement of morbidity includes two aspects – incidence and prevalence. Incidence can be obtained from “longitudinal studies” and prevalence from “cross-sectional” studies. Descriptive epidemiology may use a cross-sectional or longitudinal design to obtain estimates of magnitude of health and disease problems in human populations.

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a. Cross-sectional study is based on a single examination of a cross-section of population at one point in time.For eg. In a study of hypertension, we collect all data during the survey about age, sex, physical exercise, body weight, salt intake and other variables of interest. Then we can determine how prevention of hypertension is related to certain variables simultaneously measured such a study tell us about the distribution of a disease in population than it is a etiology.

b. Longitudinal studies:In which observations are repeated in the same population over a prolonged

period of time by means of follow up examinations. This is useful.

i. To study the natural history of disease and its future outcomeii. For identifying risk factors of diseaseiii. For finding out incidence rate of occurrence of new cases of

disease in the community.Eg. The study of blood pressure level in a population will reveal the normal values rather than abnormal ones related to disease

5. Comparing with known indices:The essence of epidemiology is to make comparisons and ask questions. By

making comparison between different populations and subgroups of the same population. It is possible to arrive at clues to disease actiology.

6. Formulation of hypothesis:A hypothesis is a supposition, arrived at from observation or reflection. An

epidemiological hypothesis should specify the following

a. The population: - the characteristics of the persons to whom the hypothesis applies

b. The specific cause being consideredc. The expected outcome – the diseased. The dose – response relationship the amount of the cause needed to lead

to stated incidence of the effect.e. The time – response relationshipFor eg. “Cigarette smoking cause lung Cancer” is an incomplete hypothesis.

An improved formulation:

“The smoking of 30-40 cigarettes per day causes lung cancer in 10 percent of smokers after 20 years of exposure.

The success or failure of a research project frequently depends upon the soundness of the hypothesis

Types of Descriptive studies• Case report• Case series• Cross sectional study• Co-relation studies

Case reports and case series:

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Case report is the descriptive study of the individual in terms of a careful, detailed report of a single patient. Case series means characteristics of a number of patients with a given disease.

Advantages:• New diseases are recognized, for example, acquired Immuno deficiency

syndrome (AIDS), 5 cases of pneuno cystic carnie pneumonia.• Formulation of new hypothesis concerning possible risk factors.

Disadvantages• Case reports are based on experience of only one person• Cannot be used to best the presence of valid association• Presence of risk factors may be purely coincidental and hence unreliable

2. Cross sectional studies:These studies describe the stats of an individual with respect to the

presence or absence of exposure and disease is assessed at the same point in time in a well defined population.

Advantages: Useful for studying association. This can be done in relation to the

factors, which remain unaltered over time, such as a gender, race, blood group etc.

Prevalence of acute and chronic diseases can be assessed. In some situations it can be considered as analytic study. Useful for health administrators in assessing health status and health

care nears of the population.

Disadvantages: Causes and effect relationship cannot be establishment Generally they are not useful for testing the hypothesis.

Ex. Suppose 500 contact tuberculosis cases participate to identify mycobacterium tuberculosis infection in a selected area. They are interview tested with tuberculin skin test (TST). The results show that 130 among 260 male contacts and 80 among 240 female contacts are positive on TST.

SEX TEST +VE TEST -VE TOTALMale 130(a) 130 (b) 260

Female 82© 10 (d) 240Total 210 290 500

The most appropriate measures of risk for this study would be odds Ration (OR), which is calculated by the following formula:

Ad 130 x 160 OR = bc = 80 x 130 = 2.0

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The odds of having mycobacterium tuberculosis infection in male contacts are 2 times compared to the odds of same infection among female contacts.

3. Co-relational Econological studiesIn this the entire populations is uses to compare disease frequencies or

health related events between different groups during the same period of time or in the same population of different points in time.

For eg. Study is done to describe patterns of mortality from 35 states and which territories are correlate with per capita alcohol sales. Death rates are highest in states with the most alcohol sales, lowest in those with the least alcohol sales and intermediate in the reminder. This early observation may contribute to the formulation of the hypothesis that drinking alcohol is associated with lover cirrhosis.

Advantages: Good for investigation of rare diseases Contributes as a first step in investigating a possible exposure – disease

relationship Inexpensive Quick / easy to conduct Utilizes data already collected by private of government sector.

Disadvantages:

Ability to prove causation is rather weak Unable to link exposure with disease in particular individuals Lack of ability to control for the effects of potential confounding factors. Represent average exposure levels rather than actual individual values

Uses of descriptive epidemiologya) Provide data regarding the magnitude of the disease load and types

of diseases problems in the community in terms of morbidity and mortality rates and ratios.

b) Provide clues to disease actiology and help in the formulation of an actiological hypothesis.

c) Provide background data for planning, organizing and evaluating preventive and curative services

d) They contribute to research by describing variations in disease occurred by time, place and person.

ANALYTIC EPIDEMIOLOGY

CASE – CONTROL STUDY:

Factor(s) present Individual with particulars cases disease

Or Absent Individual without particular disease - Controls

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PROSPECTIVE (Cohort) STUDY

Individual exposed to particular Present or absence of

factor(s)

Individual unexposed to particular Particular Disease Factor(s)

TIME

FIG.1 schematic diagram of the design of case control and cohort studies.Definition:

Observation made as in descriptive studies, but analysis is made to study the relationship between disease and other condition with different variables.

Type:

a) Case control study’b) Cohort study

CASE CONTROL STUDY:Case control study is often called “Retrospective studies” are a common first

approach to test caused hypothesis.

In case control study, individual with a particular condition or disease (case) are selected for comparison with a group of individuals in whom the condition or disease is absent (control).

In short it begins with people with disease (case) and compares them with people without the disease (control).

Features:The case control study has three distinct features,

a) Both exposure and outcomes have occurred before the start of the study

b) The study proceeds backwards from effect to cause andc) It uses a control or comparison group to support or refuse an

inference.Case control studies are basically comparison studies. Cases and controls

must be comparable with respect to know “confounding factors” such as sex, occupation, social status etc.

Suspected/risk

factor

Case (Disease

present)

Control (disease

absent)Present A bAbsent C d

a+c b+d

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Eg. Our intention to test the hypothesis that “cigarette smoking causes cancer” using the case control method investigation begins byassmbling a group of lung cancer cases (a+c) and a group of suitable method controls (b+d). One then explores the past history of these two groups for the presence or absence of smoking which is suspected to be related to the occurrence of cancer lung. If the frequency of smoking a (a+c) is higher in cases than in control 61 (b+d) an association is exist between smoking and lung cancer.

Basic step:

1. Selection of cases and controls2. Matching3. Measurement of exposure4. Analysis in interpretation

1. Selections of cases and controls:The first step is to identify a suitable group of cases and a group of controls

a. DEFINITION OF A CASE:i. Diagnostic criteria: The diagnostic criteria of the disease and the stage of

disease, if anyeg. Breast cancer stage I to be included in the study must be specified

before the study is undertaken

ii. Eligibility criteria: A criterion customarily employed is the requirement that only newly diagnosed cases within a specified period of time are eligible than old cases or cases in advanced stages of the diseases.

b. SOURCES OF CASES:The cases may be drawn from

hospital ii general population

i. Hospitals : It is often convenient to select cases from hospitalii. General population: In a population based disease occurring within

a defined geographic area during a specified period of time are through a survey, a disease register or hospital network.

B. SLECTION OF CONTROLS:

i. Hospital controls:The controls may be selected from the same hospital as the cases. But with

different illness other than study disease.

Eg. If we are going to study cancer lungs, the control group may be comprise patient with cancer breast.

ii. Relatives:The controls may also be taken up from relatives

iii. Neighborhood controls:

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The controls may be drawn from persons living in the same locality as cases, persons working in the same factory or children attending the same school.

iv. General population:Population controls can be obtained from defined geographic areas.

II. MATCHING:Matching is defined as the process by which we select controls in such a way

that they are similar to cases with regard to certain pertinent selected variables (eg. Age) which are known for influence the outcome of disease and which, if not adequately matched for comparability could distort or confound the results.

A confounding factor “is defined a one which is associated both with exposure and disease, and is disturbed unequally in study control groups more specifically a confounding factor” is one that although associated with “exposure” with investigation, is itself, independently of any such association, a “risk factor” for the disease.

a. In the study of the role of alcohol in the actiology of oesophageal cancer, smoking is a confounding factor because Is associated with consumption of alcohol It is an independent risk for oesophagel cancer. In this condition

smoking is neutralized by matching.

III. MEASUREMENT OF EXPOSURE:This may be obtaining by interviews by questionnaires or by studying past

records of cases such as hospital records employment records etc.

IV. ANALYSIS:The final step is analysis to find out

a. Exposure rates among cases and controls to suspected factorsb. Estimation of disease risk associated with exposure.

a. EXPOSURE RATES:A case control study provides a direct estimated of the exposure rate to a

suspected factors in disease and non disease groups.

Eg. A case control study of smoking and lung cancer.

Cases (with

lung cancer)

Control (without

lung cancer)Smokers (less than 5

cigarette per a day

83

(a)

55

(b)Non – smokers 2

©

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(d)

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Total (a+c) (b+d)

Exposure rates:Cases = a/(a+c) = 33/35 = 97.2 percentControls = b/(b+d) = 55/82 = 67.0 percent

This shows that the frequency rate of lung cancer was definitely higher among smoker than non smoker. The next step will be to ascertain whether there is a statistical association between exposure status and occurrence of lung cancer.

The particular test of significance will depend upon the variables under investigation. If we are dealing and disorder variables. As in the present cases the result are usually presented rates of prepositions of those present or absent in the study and in the control group.

b. Estimation of Risk:The second analytical step is estimation of disease risk associated with

exposure.

Incidence among exposed

Relative risk = Incidence among non exposed

a c÷

(a+b) (c+d)

A typical case control study does not provide incidence rate from which relative risk can be calculated directly because there is no appropriate denominator or population at risk to calculate these rates.

BIAS IN CASE CONTROL STUDIES:1. Bias due to confounding2. Memory (or ) recall bias : when cases and controls are asked

questions about their past history it may be more likely for the cases to recall the existences of a certain event of factors than the controls who are health persons

3. Selection bias: The cases and controls may be systematic differences in characteristics between cases and controls

4. Berkesonrian bias: The bias arises because of the different rates of admission to hospitals for people with different diseases.

5. Interviewer’s bias: Bias may also occur when the interviewer know th hypothesis and also know who the cases are

ADVANTAGES:1. Case control studies are specially suited for rate disease or those

with lung latency period2. Moderate ability to prove causation3. Less time consuming4. Relatively cost effective

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5. Can be done with fewer study subjects6. No risk to subjects7. Allows the study of several different etiological factors8. No attribution problems, because cases control studies do not require

follow up of individuals into the future.9. Ethical problems minimal

DISADVANTAGES:1. Probability of bias2. Selection of an appropriate control group may be difficult3. We cannot measure incidence and can only estimate the relative risk4. Do not distinguish between causes and associated factors5. Not suited to the evaluation of therapy or prophylaxis of disease 6. Another major concern is the representative new of cases and

controls.

COHORT STUDYCohort study is another type of analytical (observational) study which is

usually undertaken to obtain additional evidence to refuse or support the existence of an association between suspected cause and disease

The distinguishing features of cohort studies are:

a. The cohorts are identified prior to the appearance of the disease under investigation

b. The study groups, so defined, are observed over a period of time to determine the frequency of disease among them

c. The study proceeds forward from cause to effect

Concept of cohort:In epidemiology, the term “cohort” is defined as a group of people who share a common characteristics or experience within a defined time period (eg. age, occupation, exposure to a drug or vaccine, pregnancy insured persons etc.)

Indications of a cohort studies:

Cohort studies are indicated:

a. When there is good evidence of an association between exposure and disease as derived from clinical observations and supported by descriptive and case control studies

b. When exposure is rate but the incidence of disease high among exposed eg. Special exposure groups like those in industries exposure to Y rays etc.

c. When attribution of study population can be minimized eg. Follow up is easy, cohort is stable, co-operative and easily accessible and

d. When ample funds are available.

TYPES OF COHORT STUDIES:

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Three types of cohort studies have been distinguished on the basis of the time of occurrence of disease in relation to the time at which the investigation is intiated and continued:

1. Prospective cohort studies2. Retrospective cohort studies3. A combination of retrospective and prospective cohort studies.

1. Prospective cohort studies:A prospective cohort study (or “current” cohort sutyd0 is one in which the

outcome (eg. Disease) has not yet occurred at the time the investigation begins. Most prospective studies, studies begin in the present and continue into future.

Eg. The long-term effects of exposure to uranium was evaluated by identifying a group of uranium mineral and a comparison group of individuals not exposed to uranium mining and assessing subsequent development of lung cancer in both the groups.

2. Retrospective cohort studies:A retrospective cohort study (or “historical” cohort study) is one in which the

outcome have all occurred before the start of the investigation. The investigator goes back in time, sometimes 10 to 30 years, to select his study groups from existing records past employment, medical or other records and traces them forward through time from a past date fixed on the records, usually ipt to the present.

3. Combination of retrospective and prospective cohort studies:In this type of study, both the retrospective and prospective elements are

combined. The cohort is identified from past records and is assessed up date for the outcome. The same cohort is followed up prospectively into future for further assessment of outcome.

Elements of cohort study

The elements of a cohort study are 1. Selection of study subjects2. obtaining data on exposure3. selection of comparison groups4. Follow up5. Analysis

1. Selection of study subjects:The subjects of a cohort study are usually assembled in one of two ways-

either from general population or select groups of the population that can be readily studied.

a) General population: when the exposure or cause of death is fairly frequent in the population. Cohort may be assembled from the general population.

b) Special groups: These may be special groups or exposure groups that can readily be studied.

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Eg. Doctors, nurses, lawyers, teachers etc.

2. Obtaining data in exposure:Information about exposure may be obtained directly from the

a) Review of recordsb) Medical examination or special testsc) Environmental surveys

3. Selection of comparison groups:There are many ways of assembling comparison groups

a) Internal comparisons: In some cohort studies, no outside comparison group is required.

b) External comparisons: when information on degree of exposure is not available, it is necessary to put up an external control, to evaluate the experience of the exposed group.

c) Comparison with general population rate: If none is available, the mortality experience of the exposed group is compared with the mortality experience of the general population in the same geographic area as the exposed people.

4. Follow up:One of the populations in cohort studies is the regular follow up of all the

participants

5. Analysis;The data are analyzed in terms of

a) Incidence rates of outcome among exposed and non exposedb) Several possible outcomes related to exposure can be studied

simultaneouslyc) Cohort studies provides direct estimate of relative riskd) Close response ratios can also be calculatede) Since comparison groups are formed before disease develops, certain

forms of bias can be minimized.Disadvantages:

a) Cohort studies involve a large number of peopleb) It takes a long time to complete the study and obtain resultsc) Certain administrative problems such as loss of experience staff loss

of funding and extensive record keeping are inevitable.d) It is not unused to lose a substantial proportion of the original cohort

they may migrate lost interest in the study or simply refuse to provide any required information.

e) Selection of comparison groups which are representative of the exposed and unexpected segments of the population is a limiting factor.

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f) There may be changes in the standard methods of diagnostic criteria of the disease over prolonged follow up.

g) Cohort studies are experience.h) The study itself may alter people’s behaviour.i) With any cohort study subjects to stop or decrease smoking.j) Finally, in a cohort study practical considerate dictate that we must

concentrate on a limited number or factors possibly related to disease outcome.

EXPERIMENTAL STUDIESAn experimental study is most definitive tool for evaluation of clinical

research. It is a gold standard for evaluating effectiveness as well as side effects for therapeutic, preventive and other measures in clinical medicine as well as in public health.

Definition:An experimental study is defined as a study comparing the effect and value

of intervention against a control in a group of subject the basic different between observational and experimental study is the intervention. It may be mentioned that phase-III trails are actual clinical trails having controls groups.

Phase-I trail: These usually constitute the first step towards clinical experimentation and

research into new or improved drugs etc. animal experiments are also part of phase-I trail.

Advantage:Cheap and less time consuming

Purpose:

The adverse effect of drugsBenefits of drugs, absorption, excretion, metabolism of drugs.

Phase-II trails (Quasi-experimental design):These generally constitute the second step during drug research. They may

or may not have a control local group. They are expensive than Phase-I trails but are still less time consuming and yield better results.

Limitation can’t control for observed assess or bias and bias due to sampling variation.

Purpose: Purpose of study is benefits of drugsTrue effect of drugs

Phase-III trails:There is a clear control group similar to the experimental group. The term

of follow up and all other conditions are kept similar for the two groups.

Blinding preferably double blinding is observed to minimize bias.

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The subjects are randomly allocated to the treatment of the control group as per predetermined randomization procedure.

Types of experimental studies:These may be as follows:

1. Preventive or prophylactic trails2. Therapeutic or clinical trails3. Community trails

1. Preventive or prophylactic trails:Here intervention takes place before the disease has occurred.

Eg. Study of vaccines or risk factors.

2. Therapeutic or clinical trails:A clinical trial is an experiment with patients or subjects hence the unit of

study is a patient. The goal is to evaluate one or more new treatment for a disease or condition.

Types:

1. Randomized controls studies2. Non randomized concurrent control studies3. Cromover design4. Historical control studies5. Withdrawal studies6. Factoral design7. Group allocation design8. Studies of equivalency

3. Community trial:They involve interventions in a community wide basis. Here, the unit of

study is community.

Ex. Fluoridation trial for prevention of tooth decay. Deworming trials for ascariasis.

Summary:So far we discussed descriptive, analytical case control and cohort studies

and experimental epidemiology.

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Bibliography:1. Mahajan B.K. (2005), “Text book of preventive and social medicine”,

3rd edition, Jaypee publishers. Page No.608-6122. Park K. (2005), “Preventive and social medicine”, 18th edition,

Bhanot publications, P.No.77-813. Basavanthappa B.T. (1999), “Community health Nursing”, Jaypee

publishers, P.No.358-3614. Marcia Stanhope and Jeanette Lancaster (1992), “Community health

nursing”, 4th edition, Mosby publications, Indore, P.No.222-224.5. Swarnkar Keshav (2006), “Community health nursing”, 2nd edition,

N.R. Brothers Publications, Indore, P.No.61-63.

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