Case controlstudyi
- 1. Case-Control study Dr Paul T Francis, MD Com Med College of Medicine, Zawia
- 2. Contents Introduction Design() of study Selection of cases and controls Calculation of Exposure rates Calculation of Odds ratio Calculation of ‘P’ value
- 3. Problem () 200 students appeared for Exam() Only 90 passed, 110 failed Problem – Why many students failed?
- 4. What is the reason ()? Analysis – Possible() reasons for failure 1. Question paper difficult - No 2. Lecture taken – Yes 3. Attended lecture – All students did not attend. How do we know not attending the lecture is the correct() reason?
- 5. Case Control study Reason= Failed in exam (Problem +) Pass (Problem -) Reason + (absent from lecture) a b a+b Reason ‘–’ (present for lecture) c d c+d a+c b+d
- 6. Failed in exam (problem+) Pass (Problem-) Reason ‘+’ (absent from lecture) a+b Reason ‘–’ (present for lecture) c+d 110 (a+c) 90 (b+d) 200 100 (a) 80 (d) 10 (b) 10 (c)
- 7. Analysis Odds ratio = axd/bxc 100x80/10x10 = 80 Students not attending lecture has 80 times more chance() of failing in the exam than those who attend the lecture.
- 8. This result is not by chance() because P-value is < (less than) 0.05
- 9. Seal A sea animal that eats fish and lives around coasts.
- 10. Phocomelia
- 11. Phocomelia In 1950 many children in Europe were born with Phocomelia Doctors were worried() . Why is this happening()? They asked mothers of these children Was there any problem during pregnancy? No Did they suffer from any disease? No Did they take any medicine? Yes – Thalidomide for morning sickness
- 12. Thalidomide tragedy They did a Case-Control study to find whether it is the reason Case-Control study proved() that Thalidomide was the cause Thalidomide was banned()
- 13. Analytical epidemiology We test whether there is an association(, ) between a disease and the suspected() factor We also measure the strength() of association
- 14. Case Control study Sometimes called ‘retrospective () study’ Is the first step() to test hypothesis Both cause() and outcome (disease) have occurred before doing the study Proceeds() backwards from effect to cause
- 15. Case Control study There is a control or comparison() group to test the hypothesis This is the most important feature of Analytical epidemiology
- 16. Design of a Case Control study Cases (Disease+) Control (Disease-) Risk factor present(+) a b Risk factor absent (-) c d a+c b+d
- 17. Design of Case Control study a b c d a+c b+d Cases (Disease +) Control (Disease -) Risk factor + Risk factor -
- 18. Steps in a Case Control study Selection() of cases and controls Matching Measurement of exposure to risk factor Analysis and interpretation
- 19. Selection of Cases Cases selected should have the correct diagnosis Only cases with the confirmed() diagnosis should be included
- 20. Selection of Controls Controls must be FREE from the disease under study. If there are sub-clinical cases, do laboratory test to make sure that the person has no disease
- 21. Sources of controls Hospitals (patients having other disease) Neighborhood controls General population How many controls will you take for a case? In large studies generally 1 In small studies (below 50) up to 4
- 22. Matching Matching is a process by which we select controls in a such a way that they are similar() to cases in important variables Age, Sex, Occupation etc.() By matching we can neutralize() any confounding() factor()
- 23. Matching - examples For studying Lung cancer the Controls should be males and not females For studying Lung cancer the Controls should be adult males and not small boys For studying Breast cancer the controls should be females and not males! For studying Breast cancer the controls should be adult females and not small girls
- 24. Measurement of exposure to cause There must be a clear Definition for the risk factor. That should be same for Cases and Controls E.g. Smoking- number of cigarettes, duration of smoking, type of cigarette etc.
- 25. Analysis Calculate exposure rates among cases and controls Calculate the disease risk associated with exposure (Odds ratio)
- 26. Cigarette smoking and Lung cancer Descriptive epidemiology of Lung cancer patients was done and the following are the important characteristics() Males Cigarette smokers Hypothesis is ‘cigarette smoking is the cause for lung cancer’
- 27. Analysis Exposure rate to smoking Cases = a/a+c 33/35 = 94.2% Controls = b/b+d 55/82 = 67% Cases (Lung cancer +) Controls ( No lung cancer) Smoking + Smoking - 33(a) 55(b) 2(c) 27(d) 35(a+c) 82(b+d)
- 28. Estimation of risk Those who are having lung cancer are smoking more(94.2%) However it does not mean that 94.2% of all smokers will develop lung cancer. We estimate risk to develop lung cancer in smokers by calculating ‘Odds ratio’
- 29. Odds ratio Odds ratio = ad/bc 33x27/55x2 = 8.1 Those who smoke have 8.1 times the risk of developing Lung cancer than those who do not smoke If the odds ratio is 1 means no risk
- 30. P- value We have found cigarette smokers has 8.1 times more risk of getting Lung cancer There are thousands of Lung cancer patients in the world We have taken only a small() sample() of 35 cases How do we know it is true for all lung cancer patients?
- 31. P-value To see if this association is due to chance() It is the probability() that the difference is due to chance If P value is <0.05 it is considered statistically significant() P value in lung cancer study is <0.001
- 32. Analysis
- 33. CC study - advantages Easy to conduct() Inexpensive () No risk() to people No attrition (loss of patients) problems No ethical() problems
- 34. CC study - disadvantages Problem of accuracy() of data Loss of memory() How many cigarettes a person smoked 20 years ago? Incomplete() records What medicine a lady took in pregnancy? Getting good controls is difficult()
- 35. Summary Case Control study is used to test hypothesis It involves four steps Selection of cases and controls Matching Measuring exposure Analysis (Exposure rate, Odds ratio and P value)
- 36. Summary The analysis of Case Control study is by a 2x2 design Exposure rates are calculated among cases and controls Dis + Dis - RF + a b RF - c d a+c b+d
- 37. Summary Odds ratio is calculated to estimate the risk of disease among those who are exposed to the cause P value is calculated to know whether the difference is statistically significant Case Control study helps us prove the cause of disease