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Case control & cohort studies

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Presentation on theme: "Case control & cohort studies"— Presentation transcript:

1 Case control & cohort studies
Dr. Asif Rehman

2 Epidemiological study designs
All Studies Experimental Observational RCT Community Trail Descriptive Analytical Cross sectional Surveys Qualitative Case Control (Cross sectional survey) Cohort

3 Case control & cohort studies
Analytical studies (Case control & cohort studies): In contrast to descriptive studies that look at entire populations, in Analytical studies, the subject of interest is the individual within the population. The object is not to formulate but to test hypothesis. For each of these studies one can determine whether or not a statistical association exist b/w a disease and suspected factor. And if one exist, the strength of the association.

4 Case control & cohort studies
Case-control studies; Are often called Retrospective study. Are first approach to test casual hypothesis. Its involves two groups cases & controls and basically a comparison study. Cases and control must be comparable with respect to known “Confounding Factors” e.g Age, Sex Occupation, Social status The focus is on a disease or some other health problem that has already developed.

5 Case control & cohort studies
Case-control studies; The questions asked relate to personal characteristics and related exposure which may be responsible to the condition studied. Its has three distinct features; Both exposure and outcome have occurred before the start of the study The study precede backward from effect to cause It use a control or a comparison group to support or refute an inference

6 Case control & cohort studies
Case-control studies: There are four basic steps in conducting a case-control study Selection of cases and control Matching Measurement of exposure Analysis and interpretation

7 Case control & cohort studies
Case control study Example: A pediatrician researcher wants to study the association between the use of tape water for drinking and diarrhea. His hypothesis is that “children using tape water for drinking are more likely to suffer from diarrhea” as compared to those who use mineral water. In this example children who are suffering from diarrhea will be cases while those not having diarrhea will be controls. The exposure in this study is the use of tape water for drinking while the outcome is diarrhea. Cases and controls are questioned, or their medical record are consulted regarding past exposure to risk factors. Later measure of association is determined which in case of a case-control study is “Odd Ration” (K Park, 20th edition)

8 Case control & cohort studies
Odd Ratio (Cross product ratio): From a typical case-control study, we can derive what is known as “Odd ratio” which is a measure of the strength of the association between risk factor and outcome. O.R is a key parameter in the analysis of case-control study. The derivation of O.R is based on three assumptions: The disease being investigated must be relatively rare. The Case must be representative of those with disease. The Control must be representative of those with no disease. O.R = ad/bc

9 Case control & cohort studies
Odd Ratio Calculation: A study looking at the lung cancer in men compared cases with non cases and found that 33 out of 88 people who smoke develop lung cancer. While 2 out of 29 people who didn’t smoke also developed lung cancer. Since it is a case control study, we will calculate OR. Disease No Disease Exposed (smoking) a (33) b (55) Not Exposed (No smoking) c (2) d (27)

10 Case control & cohort studies
Odd Ratio Calculation: OR = ad/bc OR = 33 x 27/ 2 x 55 OR = 8.1 Interpretation; smokers showed a risk of having lung cancer 8.1 times that of non smoker or smokers are 8.1 times more likely to develop lung cancer than a non smokers. Disease No Disease Exposed (smoking) a (33) b (55) Not Exposed (No smoking) c (2) d (27)

11 Case control & cohort studies
Case-control study Advantages: Inexpensive and relatively easy to carried out Required comparatively few subjects. Take lesser time as compared to other analytical studies Recommended for studies on rare disease. No risks to the subjects Risk factors can be identified. Rationale prevention and control program can be established No need of follow up

12 Case control & cohort studies
Case-control study Disadvantages: Recall bias is the main problem as the cases will be more likely to recall the past exposure. Selection bias is another problem if the cases and control are not properly selected. We can not measure the incidence.

13 Case control & cohort studies
Another type of analytical study usually undertaken to obtain additional evidence to refute or support the existence the of an association b/w suspected cause and disease. Also known by the name of Prospective study, Longitudinal study, Incidence study. In epidemiology the term cohort is defined as a group of people who share a common characteristics or experienced with in a defined period of time.

14 Case control & cohort studies
Distinguishing features of cohort studies are; The study proceed forward from cause to effect. The cohorts are identified prior to the appearance of a disease under investigation. The study groups are observed over a period of time to determine the frequency of disease among them

15 Case control & cohort studies
Indication for cohort study; Indicated when there is a good evidence of an association b/w exposure and disease. When exposure is rare , but the incidence of a disease high among exposed e.g exposure to x-rays in industries. When cohort is stable, co operative, accessible and follow up is easy When enough funds are available.

16 Case control & cohort studies
Types of Cohort studies; Prospective cohort studies. Retrospective cohort studies. A combination of Prospective and Retrospective cohort studies.

17 Case control & cohort studies
Types of Cohort studies; Prospective cohort Defined population Retrospective cohort Exposed Non exposed (Disease) (No disease) (Disease) (No disease) 2005

18 Case control & cohort studies
Elements of Cohort studies; Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis

19 Case control & cohort studies
Relative Risk; RR is the ratio of the incidence of the disease among exposed and the incidence among non exposed. It is a direct measure of the strength of association b/w cause and effect. RR = a/a+b ÷ c/c+d RR of 1 indicates no association, RR greater than 1 suggest positive association and RR less than 1 indicates negative association b/w exposure and disease. Disease No Disease Exposed (Smoking) a b Non Exposed (Non Smoking) c d

20 Case control & cohort studies
Relative Risk calculation; A 5 years cohort study in which the evidence of disease in exposed was compared with that of unexposed. RR = a/a+b ÷ c/c+d RR = 70/7000 ÷ 3/3000 RR = 10 Interpretation; A relative risk of 10 indicates positive association, it means that smokers are 10 times at greater risk of developing lung cancer than non- smokers Disease No Disease Exposed (Smoking) a (70) b (6930) Non Exposed (Non Smoking) c (3) d (2997)

21 Cohort study Advantages Incidence can be calculated
Multiple outcomes to a single exposure can be detected It helps in calculating Relative risk and Attributable risk Temporal association is best studied in prospective cohort study

22 Cohort study Disadvantages Expensive study to perform Time consuming
Required large number of people. Strict follow-up is required Not suitable for a rare disease Loss to follow-up due to migration or death of the respondent

23 Exercise A study looking at the skin cancer in women compared cases with non cases and found that 75/100 cases didn’t exposed to Sunbathing compared with 25/100 of the non cases. Name the study design? Develope 2 x 2 table Calculâtes the appropriate masure Interpret the result.

24 Exercise A study begun in 1965, a group of 270 adults in Manchester were asked about cigarette smoking. The occurrence of cases of cancer between 1981 and 1995 was studied. Among 120 exposed patients 80 developed cancer. while 50 individuals among 150 non exposed were disease free. Name the study design? Develop 2 x 2 table Calculate the appropriate measure Interpret the result.

25 THANK YOU

26 Case control & cohort studies
Difference between case-control & Retrospective cohort study In case-control, one starts from the outcomes i.e. cases and bases/controls/referents (matched or not), and tries to study what the exposure was. Using the 'rare disease assumption', both odds ratio and relative risk can be used to study association between exposure and outcome. However, odds ratio is preferred because the prevalence of the disease outcome is already identified.  Retrospective cohort is when one already has determined the exposure in the study cohort and tries to study the association of exposure to disease outcome within that cohort in a retrospective manner. Both OR and RR are safe to use here, as in prospective cohort. 


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