1

Presented by: reMAN dhaKALCODSH-NMCFIRST BATCH

Introduction

Types of epidemiology

Types of analytical epidemiology

Case control study

Cohort study

Comparison between case control and cohort study

2

John M. last: "the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems.

3

4

EPIDEMIOLOGY

5

Analytical epidemiology

Second major type of epidemiology. Focus on individual within population unlike descriptive epidemiology.. Objective not to formulate hypothesis but to test hypothesis.

TYPESA. CASE CONTROL STUDYB. COHORT STUDY

Retrospective or trohoc study

Distinct features:

1. Both exposure and outcome have occurred before the start of disease

2. Study proceed backward from effect to cause

3. Uses a control or comparison group to support or refute an inference.

6

7

The basic study design

Control(those without condition)

eg: those free of oral cancer

Cases(those with condition)

eg: cases with oral cancer

Unexposed (without characteristic or risk factor)

Eg. Non chewers

Exposed (with characteristic or risk factor)

Eg. tobacoo chewers

1. Selection of cases and controls

2. Matching

3. Measurement of exposure , and

4. Analysis and interpretation.

8

9

A. Selection of case

Definition of a case:

I). diagnostic criteria:

ii). Eligibility criteria

Sources of cases:

i). Hospital

ii). General population

B. Selection of control

Sources of controls:

i). Hospital controls(common

source of selection bias)

ii). Relatives

iii). General population

Number of

controls/control

groups

Define as:”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 to influence the outcome of disease and which, if not adequately matched for comparability, could distort or confounded the result”.

CONFOUNDING FACTOR

10

EXPOSURE(eg. Consumption

of alcohol)

DISEASE(eg. Oesophageal

cancer)

CONFOUNDING FACTOR (eg. smoking, age)

Definition and criteria about exposure are just as important as those used to define cases and controls. This may be obtained by :

Interviews

Questionnaries

Studying past record of cases such as hospitalrecords, employment records etc.

Clinical or laboratory examination.

Investigator should not know whether a subject is incase or control group.

11

The final step is analysis, to find out:

a) Exposure rates among cases and controls to suspected factors

b) Estimation of disease risk associated with exposure (ODD RATIO)

12

Tobacco

chewers

Non-chewers

Cases

(with oral cancer)

33 (a)

2 (c)

Controls

(without oral cancer)

55(b)

27(d)

13

Total 35 (a +c) 82 (b +d)

Example of case control study of tobacco chewing and oral cancer

EXPOSURE RATESa. Cases =a/(a+c) = 33/35 = 94.2%b. Controls =b/(b+d) = 55/82 = 67.0%

1. Relative risk = Incidence among exposed

incidence among non exposed

= a c

(a+b) (c+d)

2. Odds ratio (OR) = (a/b)

(c/d)

It measure strength of the association between risk factor and outcome. ,

1. Selection bias :

special types:

a) Prevalence incidence (selective survival)

b) Admission rate ( Berkson/Berkesonian)

2. Information bias:

a) Memory or recall bias

b) Telescopic bias

c) Interviewer’s bias

3. Bias due to confounding

15

16

ADVANTAGES:

1. Relatively easy to carry out.2. Rapid and inexpensive3. Require fewer subjects.4. Suitable for investigation of

rare diseases.5. No risk of subject.6. Allows the study of several

different etiological factors.7. Risk factor can be identify8. No attrition problem

because do not require follow up.

9. Minimal ethical problem.

DISADVANTAGES:

1. Problem of bias since it relies on past memory or past records.

2. Difficulty in selection of appropriate control group.

3. Can not measure incidence only RR.

4. Doesn’t distinguish between cause and associated factors.

5. Not suited for the evaluation of therapy or prophylaxis of disease.

CASE CONTROL STUDY

Prospective ,longitudinal, incidence and forward-looking study

Distinguishing features:

a) The cohorts are identified prior to theappearance of the disease

b) The study groups, so defined, are observedover a period of time to determine thefrequency of disease among them

c) Study proceeds from cause to effect.

17

time

Study begins here

Studypopulation

free ofdisease

Factorpresent

Factorabsent

disease

no disease

disease

no disease

present

future

When there is good association between exposure and disease.

When exposure is rare, but the incidence of disease is high among exposed.

When attrition of study population can be minimized.

19

20

1. PROSPECTIVE COHORT STUDY

1. RETROSPECTIVE COHORT STUDY

2. A COMBINATION OF PROSPECTIVE AND RETROSPECTIVE COHORT STUDY.

- Outcome has not yet occurred the time

of investigation begins.

Measure exposureand confounder

variables

Exposed

Non-exposed

Outcome

Outcome

Baseline

time

Study begins here

Outcomes have all occurred before the start of the investigation.

22

Measure exposureand confounder

variables

Exposed

Non-exposed

Outcome

Outcome

Baseline

time

Study begins here

1. Selecting of study subject

2. Obtaining data on exposure

3. Selection of comparison group

4. Follow up

5. analysis

23

24

1. Selecting of study subject

When exposure or cause of death is fairly frequent in

the population

i. Select group –Professional

group ( doctors,nurses )

ii. Exposure group-High risk situation

(eg.radiologist exposed to x-ray)

Obtaining data on exposure

Information about exposure may be obtained directly

from:-

25

Selection of comparison group

a. Internal comparisons:-

Comparison groups are in built (eg. Smoking, bp etc.) within

same cohort group.

b. External comparisons:- Eg. Smoker and non smoker,

radiologists with opthalmologists.

c. With General population:- If none is available, mortality of

exposed group with general population

Follow up

Main problem Procedures to obtain data

for assessing the outcomeare:

a. Periodic medical checkupb. Reviewing hospital recordsc. Routine surveillance of

death recordsd. Mailed questionnaries,

telephone calls, periodic home visits.

Data are analysed in term of:

a. Incidence rates of outcome among exposed and non-exposed:

26

27

RISK FACTOR (TOBACCO)

DEVELOPEDORAL CANCER

DID NOT DEVELOP

TOTAL

PRESENT (CHEWERS)

45 (a) 9955 (c) 10000 (a + c)

ABSENT(NON CHEWERS)

5 (b) 9995 (d) 10000 (b + d)

5. ANALYSISData are analysed in term of:a. Incidence rates of outcome among exposed

and non-exposed

Incidence rates:1. Among tobacco chewers: = 45/10000 =4.5 per 10002.Among non chewers = 5/10000 = 0.5 per 1000

b. Estimation of risk

A. Relative risk (RR):

= incidence of disease among exposed

incidence of disease among non-exposed

= 4.5/0.5

= 9

It implies 9 times higher risk of development of oral carcinoma in tobacco chewers compared to non chewers.

28

B. Attributable risk (AR) Or “risk difference”:

Incidence of disease rate among exposed- incidence among non exposed

Incidence rate among exposed

= 4.5 – 0.5

4.5

= 88.9%

Indicates to what extent the disease under study can be attributed to the exposure.

29

1. Selection bias:

Non consent bias

Missing data bias

2. information bias:

Error in classification of individual

Diagnostic bias

3. Confounding bias :

Due to confounding factors

4. Post hoc bias: 30

1. Incidence can be calculated

2. Several possible outcomes related to exposure can be studied simultaneously.

3. Provide a direct estimate of RR.

4. Dose response ratios can be calculated.

5. Since comparison groups are formed before disease develops, certain forms of bias can be minimized like misclassification of individual.

31

1. Unsuitable for investigating uncommon disease.

2. Long time to complete study and obtain results.

3. Administrative problem –

Extensive record keeping

4.Expensive

5. Alter people behavior

Stop or decrease smoking

Loss of interest

migration

5. Ethical problem of varying important

32

Start disease :effect cause

First approach to testhypothesis.

Involve fewer subject.

Yield result quickly.

Suitable for studying rare disease.

Gives RR only.

Relatively inexpensive. Does not give information about diseases other than that selected for the disease. 33

Start people: cause effect.

Reserved for testing precisely formulated hypothesis.

Involve larger number of subjects. Results are delayed due to long follow up. Unsuitable for studying for rare diseases.

Yield RR and AR.

Expensive Can give information more than one disease.

1

2

3

4

5

6

7

8

34