Case control study and Cohort studyNIVETHA C

OBSERVATIONAL STUDIES• DESCRIPTIVE STUDIES• ANALYTICAL STUDIES 1. Ecological 2. Cross-Sectional 3. Case- Control 4. Cohort

CASE CONTROL STUDY Retrospective studyFeatures:

1. Both exposure and outcome have occurred before the start

of the study

2. The study proceeds backwards from effect to cause

3. It uses a control or comparison group to support or refute

an inference

Basic Steps1. Selection of cases and controls2. Matching3. Measurement of exposure4. Analysis and interpretation

Selection of cases and controls• SELECTION OF CASE : The prior definition of what constitutes a case is crucial to the case control study . It involves 2 specifications 1. Diagnostic criteria 2. Eligibility criteria • SOURCE OF CASES Cases can be drawn from hospitals or general population.

• SELECTION OF CONTROLS: THE CONTROLS MUST BE FREE FROM DISEASE UNDER STUDY They must be similar to the cases as possible except for the absence of the disease under study .

SOURCE OF CONTROLS:Hospitals Relatives NeighborhoodGeneral populationOne control per case if many cases are available or as many as 2,3 , or even 4 if the study group is small

• MATCHING : 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 variable which are known to influence the outcome of disease and which if not adequately for comparability could distort of confound the result .

• A confounding factor is defined one which is associated with both associated disease and is distributed un equally in study and control groups .

• Types 1. Group Matching: assigning cases to subcategories based on their characteristics like agem occupation, etc. and then establishing appropriate controls. 2. Pair matching: It is finding a control for particular case as closely resembling as possible except for disease under study.

• MEASUREMNT OF EXPOSURE : Information of exposure should be obtained precisely in same manner in both the cases and controls.

It can be recorded by interview, by questionnaires or by studying past records.

• ANALYSIS : t is the final step to find out : 1. Exposure rate among cases and controls to a suspected factor. 2. Estimation of diseases risk associated with exposure

EXPOSURE RATESA case control study provides a direct estimation of the exposure to a suspected factor ion the disease and non-disease groups

Exposure rates Cases : Controls :

The next step is to ascertain weather there is a statistical association between exposure status and disease occurrence which is resolved by calculating p value.

Suspected or risk factors

Cases (disease present)

Controls (Disdease Abest)

Present a b

Absent c d

a+c b+d

ESTIMATION OF RISK : It is obtained by an index known a ‘ relative risk ‘ or odds ratio •

• Odds ratio is the measure of the strength of association between the risk factor and outcome .it based on 3 assumptions : 1. disease being investigated must be relatively rare . 2. case must be representative of those with the disease 3. controls mist be representative of those withput the disease. odds ratio = ad / bc.

ADVANTAGES

• Relatively easy to carry out• Rapid and inexpensive

compared to cohort study• Requires comparatively few

subjects• No risk to the subject• Allows the study of several

different etiological factors like effect of smoking, physical activity, diet.

DISADVANTAGES

• Problems of bias • Selection of an appropriate

control group may be difficult• Incidence cannot be measured,

only relative risk can be estimated

• Not suited for evaluation of therapy or prophylaxis of disease

COHORT STUDY:• Definition: it is a type of study usually undertaken to obtain

additional evidence to refute or support the existence of an association between suspected cause and disease.

• Characteristics 1. Cohorts are identified prior the appearance of the disease under investigation . 2. The study groups are observed over a period of time to determine the frequency of the disease among them . 3. The study proceeds forward from cost to effect

• COHORT SELECTION :Cohort is a group of peple who share a common characteristic or experience within a definite time period.• For selecting a cohort the following facts should be considered 1. Cohorts must be free form the disease under study. 2. Both the study and control cohorts should be equally susceptible to the disease under study. 3. Both the cohort must be comparable in respect to all possible variable which may Influence the frequency of the disease.

Cohort Disease Present

Disease Absent

Total

Exposed to etiological agent a b a+b

Not exposed to etiological agent c d c+d

Framework of cohort study

TYPES OF COHORT STUDY

COHORT STUDIES

PROSPECTIVE RETROSPECTIVE COMBINATION OF BOTH.

STEPS OF COHORT STUDY : • Selection of study subjects • Obtaining data on exposure • Selection of comparison groups • Follow up• Analysis

• SELECTION OF STUDY SUBJECT : Subjects of a cohort study are usually assembled from general population or special groups .

• OBTAINING DATA ON DATA EXPOSURE : Information about exposure is obtained directly from any one or all of the following sources : Cohort members , review of records , medical examinations or special tests and environmental surveys

• SELECTION OF COMPARISON GROUPS : There are many ways of assembling a group .1. Internal comparison : when a single cohort enters the study, on the basis of information obtained, be classified into several comparison groups according to the degrees of levels of exposure to risk before the development of disease in question.2.External comparison : when the degree of comparison is not available , it is necessary to put up an external , to evaluate the experience of the exposed group .3.Comparison with general population rates .

4. FOLLOW UP COMPRISES OF: 1.Period medical examination of each member if the cohort 2.Reviewing physician and hospital records 3.Routine surveillance of death records 4.Mailed questionnaires , telephone calls and periodic home visits. • Limitation : A certain percentage of losses to follow up are inevitable due to death , change of residence , migration , or withdrawal of occupation . These losses may bias the result .

• ANALYSIS: - Incidence rates - Estimation of risks

• INCIDENCE RATES: Among cases: Among controls : • ESTIMATION OF RISK -Relative risk -Attributable risk -Absolute risk

• Relative Risk: It is the direct measure of strength of the association between suspected cause and effect.• Relative risk:

Interpretation: I exp= I nonexp = no correlation I exp> I nonexp = Positive association I exp < I nonexp = negative correlation

• Attributable Risk: It is the difference in incidence rates of disease between an exposed group and non exposed group

• = x100• It signifies potential for prevention.

• Absolute risk:

Risk of developing the disease, irrespective of exposure to risk factor and is expressed in percentage.

• = x100

ADVANTAGES• Incidence can be

calculated • Several possible outcomes

related to exposure can be studied simulateously • Provide a direct estimation

of relative risk• Dose-response ratios can

also be calculated

• DISADVANTAGES• Involve a large number of

people• General unsuitable for

investigating uncommon disease or diseases with low incidence in the population• Long time to complete

study• Loss of substantial

proportion of cohort due to migration, loss of interest.• Expensive

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