KNOWLEDGE FOR THE BENEFIT OF HUMANITYKNOWLEDGE FOR THE BENEFIT OF HUMANITY

PUBLIC HEALTH AND EPIDEMIOLOGY (HFS3063) Epidemiological Study Designs:

CASE CONTROL

Dr. Dr. MohdMohd RazifRazif ShahrilShahril

School of Nutrition & Dietetics School of Nutrition & Dietetics

Faculty of Health SciencesFaculty of Health Sciences

UniversitiUniversiti Sultan Sultan ZainalZainal AbidinAbidin

1

Topic Learning Outcomes

By the end of this lecture, students should be able to;

• describe case control study design.

• explain the advantages and disadvantages of case control study design.

2

Case-control studies • Purpose is to establish association between exposure to

risk factors and disease.

• Members of the population with the disease are selected into the study at the outset and risk factor information is collected retrospectively – Known as CASES

• A second group of individuals who do not have the disease is also included in the study – Known as CONTROLS

• Often used in the study of rare disease or preliminary study – Where little is known about the association between the risk

factors and disease of interest

3

(cont.) Case control studies

4

(cont.) Case control studies

5

(cont.) Case control studies • Case-control studies are prone to bias and confounding

• To minimize bias care must be taken in

– the selection of cases and control

– establishing definitions of disease, risk factors

– ensuring there are no confounding associations between

detection of disease and risk factors exposure.

6

Choice of cases • Care must be taken when choosing cases for the study.

• It is important to distinguish between stages or subtypes

of disease and to define a measure of health status

– E.g. when studying physical activity it is important to define what

is meant by physical activity in terms of types, nature and level.

• It is also important to establish whether interest is in

– incident cases, subjects entered into study on detection of

disease, OR

– prevalent cases, those who have been diagnosed as having the

disease prior to the study

7

(cont.) Choice of cases • Views, behavior and reports of exposure to risk factor

amongst incident cases and prevalent cases will tend to

differ

– Those diagnosed previously are likely to be more informed about

the disease and may have altered their behavior and attitudes

• Incident case design is preferred as it reduces recall bias

and over-representation of cases with long standing

base.

8

Choice of controls • Controls should

– come from the same population at risk of disease

– not have the disease

– be representative of the target population

• Selecting controls often proves harder than cases and

requires great care in the prevention of bias.

• A sampling frame of hospital patients is often used to

select controls

– however risk factors such as diet and smoking are commonly

linked to many diseases

9

(cont.) Choice of controls • Selecting controls from hospital patients sampling frame

might therefore over-estimate population exposure

– underestimation of association between disease and exposure

– to overcome this; use more than one control group

• Multiple controls can be used for each case

– Giving the study greater power

– Particularly where the number of cases is small due to the

disease being rare.

10

Exposure to risk factors and matching

• Exposure measurements are reliant on memory

(interviewed retrospectively) and/or medical records.

• Exposure estimates are vulnerable to recall bias

– Those with disease are more likely to remember exposure than

those without

• Exposure estimates are vulnerable to interview or

measurement bias

– Interviewer interviews or reports findings systematically

differently between cases and control and confounding factors

– Can be overcome by including blinding in the design so that they

do not know who is a case and who is a control at the time of

interview.

11

(cont.) Exposure to risk factors and matching

• Confounding factors must be identified prior to the start

of study.

• Individuals matched to controls where it is thought that

other factors, aside from those risk factors of interest,

might contribute to the development of disease

– E.g: age, sex,

• The factor upon which cases and controls are matched

can not be studied as risk factor.

• Alternative method of overcoming confounding is to

collect relevant information on them and adjust

statistically.

12

(cont.) Exposure to risk factors and matching

• Matching is used in case-control studies for;

– To permit allowance for confounders which are complex of

difficult to define

• e.g. by comparing within identical twin case/control pairs it is possible to

allow for ill-defined genetic confounders.

– To make allowance for confounders statistically more efficient.

Efficient analysis requires that there be a similar ratio of cases to

controls at each level of exposure to the confounding variable

– To reduce biases in the ascertainment of exposure.

• Egg. Data collected from deceased cases should be matched to deceased

control as information is collected from their family members.

13

Analysis of data

• Case control study measure the odd of exposure based

on disease

– Compared to cohort study which measure relative risk of disease

based on exposure

• Odds ratio has two components;

– The odds of exposure for cases

– The odds of exposure for controls

14

(cont.) Analysis of data

• The odds of exposure for cases

– Number of cases exposed / number of cases unexposed given

by Odds / cases = a / b

• The odds of exposure for controls

– Number of controls exposed / number of controls unexposed

given by Odds / controls = c / d

• The estimated Odds Ratio is then Odds = ad / bc

15

(cont.) Analysis of data

• The Odds Ratio is interpreted as

– OR < 1; Odds of exposure for cases are less than those for

control. Exposure appears to reduce risk of disease.

– OR = 1; Odds of exposure for cases are the same as those for

control. Exposure does not appear to be a risk factor.

– OR > 1; Odds of exposure for cases are more than those for

control. Exposure appears to increase risk of disease.

• A 95% confidence interval (95% CI) gives an indication

of the confidence we have in the estimated Odds Ratio

– E.g. if the entire 95% CI is above 1, it is concluded that exposure

significantly increases the risk of disease at the 95% level.

16

(cont.) Analysis of data

• Assessment of whether an observed association is likely

to be directly causal and not the results of unrecognized

confounding depends on;

– The size of the relative risk – higher relative risk are less likely to

be explained by unknown confounders.

– The presence of a dose-response relation – the observation of a

higher risk in subjects with a greater exposure to the risk factor

favors a direct causal relationship.

– The existence of plausible biological mechanism which might

explain a causal relationship.

17

Advantages of case control studies

• Quick

• Cheap

• Particularly suited to the study of rare diseases as the

diseased are selected at the outset of the study.

18

Disadvantages of case control studies

• Difficulties in overcoming potential bias and confounding.

• The successful selection of both cases and controls who

are representative of their respective population is often

difficult.

• An inability to infer causality and no information on the

chronology of disease and exposure.

• Inefficient in studying risk factors which are rare.

• Studies are often not population based, therefore it is

impossible to calculate incidence of disease.

19

Recapitulate In this lecture, you have been exposed to;

• definition of case-control studies

• choice of cases and controls

• exposure to risk factors and matching

• analysis of data for case-control studies

• advantages and disadvantages of case-control studies

20

Thank YouThank You

21