What is Epidemiology?What is Epidemiology?

Definitions of EpidemiologyDefinitions of Epidemiology

Lilienfeld A: in Foundations of Epidemiology

THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION

(Note that epidemiology is applied to a broad variety of health related issues – not just infectious diseases.)

Purposes Of EpidemiologyPurposes Of Epidemiology

1. Identify causes and risk factors for disease.

2. Determine the extent of disease in the community.

3. Study natural history and prognosis of disease.

4. Evaluate preventive and therapeutic measures

5. Provide foundation for public policy

What Is The Unique Skill What Is The Unique Skill Of Epidemiologists?Of Epidemiologists?

MEASURING DISEASE FREQUENCY IN

POPULATIONS

Measuring Disease Frequency Measuring Disease Frequency Has Several ComponentsHas Several Components

Classifying the disease

Deciding what constitutes a case of disease in a study

Finding a source for ascertaining the cases

Defining the population at risk of disease

Defining the period of time of risk of disease

Obtaining permission to study people

Making measurements of disease frequency

Relating cases to

population and time at risk

Case DefinitionCase Definitioncase definition -defines a case in time, person and

place. Time criteria may include all cases of a disease identified from, for

example, January 1, 2008 to March 1, 2008. Person criteria may include age, gender, ethnicity, and clinical

characteristics such as symptoms (e.g. cough and fever), clinical tests (e.g. pneumonia on chest X-ray).

Place criteria will usually include a geographical entity such as a town, state, or country but may be as small as an institution, a school class, or a restaurant meal session.

Two Broad Types of Two Broad Types of EpidemiologyEpidemiology

(Who? Where? When?) Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of person, place, and time.

Typical study design:

community health survey (approximate synonyms - cross-

sectional study, descriptive study)

(How? Why?) Testing a specific hypothesis

about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest.

Typical study designs:

-cohort study -case-control study

DESCRIPTIVE EPIDEMIOLOGY

ANALYTIC EPIDEMIOLOGY

Cross Sectional StudyCross Sectional Study

Basically this is a survey– Defines populations to be studied– Collects information from pop. members about

their disease and exposure– “snapshot” of the population – point in time

– Examines relationship between a variable and a disease – but does NOT establish cause and effect which requires data over time

The Basic Triad Of The Basic Triad Of

Descriptive EpidemiologyDescriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK

FOR IN DESCRIPTIVE EPIDEMIOLOGY:

TIME PLACE PERSON

Time Person

Place

TimeTime

Changing or stable?

Seasonal variation.

Clustered (epidemic) or evenly distributed (endemic)?

Point source or propagated.

PlacePlace

Geographically restricted or widespread (pandemic)?

 Relation to water or food supply.

 Multiple clusters or one?

PersonPersonAge

Socio-economic status

Gender

Ethnicity/Race

Behavior

Descriptive Epidemiology Is A Descriptive Epidemiology Is A Necessary Antecedent Of Necessary Antecedent Of

Analytic EpidemiologyAnalytic Epidemiology To undertake an analytic

epidemiologic study you must first: Know where to look Know what to control for Be able to formulate hypotheses

compatible with laboratory evidence

The Basic Triad Of The Basic Triad Of Analytic EpidemiologyAnalytic Epidemiology

THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:

HOST

ENVIRONMENTAGENT

AgentsAgents Microbes = Classic view – others include:

Nutrients – (ex rickets)

Poisons Allergens Radiation Physical trauma (car crashes)

Psychological experiences

Host FactorsHost Factors

Genetics

Immunologic state

Age

Personal behavior

EnvironmentEnvironmentInfluences interaction of host and agent Crowding Atmosphere/weather Modes of communication – phenomena

in the environment that bring host and agent together, such as:– Vector– Vehicle– Reservoir

Public Health Strategies…Public Health Strategies…

Depending on which public health strategy might be most effective, we might direct efforts at the specific agent (guinea worm), host ( immunization to prevent measles), or environment (sanitation improvements to prevent salmonella)

Analytical studies:Analytical studies:

Cohort Case Control

Cohort StudyCohort Study

Follows population forward in time from suspected cause to effect

Begins with a group free of Disease.– Selects population according to exposure – Not

whether they have the disease or health problem or not. Population is WELL DEFINED

See if the development of new cases differs between exposed and unexposed groups

Uses Relative Risk Calculation.

Cohort Study Cohort Study Example: A salmonella outbreak occurs at a Example: A salmonella outbreak occurs at a

company picnic….company picnic….Each attendee is questioned about the food

they ate (exposure) and whether they became ill afterward.

Relative risk for exposure can be CALCULATED (quantified) from the relationship between the exposure and outcome.

Relative Risk calculator at Medcalc/org

Case Control StudyCase Control StudyWork backwards from Effect Cause

Case subjects that have the disease are – compared to Control subjects who do not have the

disease but are part of the same at-risk population. Population is not well defined as in Cohort studies

You cannot calculate attack rates because you do not know the total number of people in the community who were and were not exposed to the source of the disease under study. Without attack rates, you cannot calculate relative risk

Comparison of the two groups is quantified by an ODDS RATIO*:

*See below in notes::.

Odds Ratio Calculator (Medcalc.org) The odds ratio is used frequently in

case/control studies • In a case/control study, ill persons’ (“cases”)

characteristics and exposures are compared with those of well persons (“controls”) selected from the same population in which the outbreak occurred.

– Example:

– in an outbreak suspected to stem from exposure to contaminated food at a restaurant, the ill persons’ food selections could be compared with those of some well people who also ate at the restaurant the same day. This could be done even if we didn’t know exactly how many people ate at the restaurant that day.

• A two-by-two table is constructed – The OR is calculated by multiplying across the

cells.

The formula for OR is: The formula for OR is: Odds Ratio = Odds Ratio = ad ad bc bc Where

a = number of persons with disease and with exposure of interest

b = number of persons without disease, but with exposure of interest

c = number of persons with disease, but without exposure of interest

d = number of persons without disease and without exposure of interest

a+c = total number of persons with disease (“cases”)

b+d = total number of persons without disease (“controls”)

Number of Cases of Disease X by Exposure Number of Cases of Disease X by Exposure

History, Smallville, 2004 Disease X History, Smallville, 2004 Disease X

Yes No Total

Ate Tuna Casserole a =46 b= 25 71 Didn’t Eat Tuna c= 18 d= 40 58

CasseroleThe OR in this example is: Odds Ratio = 46 x 40 = 1840 = 4.1

25 x 18 450 So those who became ill were 4.1 times as likely to have eaten the tuna

casserole. We should probably look a little more deeply into the tuna casserole!

Men Women Children Total

1st class 67% 3% 0 38%

2nd class 92% 14% 0 59%

3rd class 84% 54% 66% 62%

Total 82% 26% 48% 62%

EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC

CHARACTERISTIC

DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE

WHAT CAUSE OF DEATH IS THIS?

The previous slide shows death The previous slide shows death rates by class of ticket on the rates by class of ticket on the

Titanic, a large ocean liner that Titanic, a large ocean liner that sank after colliding with an sank after colliding with an

iceberg in 1912 iceberg in 1912