Epidemiological Exercises on case control studies
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This document discusses case-control study design and calculating odds ratios. It provides examples of 3 case-control studies examining suspected risk factors for cervical cancer, lung cancer, and esophageal cancer. For each study, it constructs a 2x2 contingency table and calculates the odds ratio to assess the strength of association between the disease and suspected risk factor. Odds ratios greater than 1 indicate the exposure increases disease risk.
Epidemiological Exercises on case control studies
- 3. 2 × 2 Contingency table 6-Mar-19Epidemiological Exercises3 Exposed to suspected RF Disease under investigation Total Present Absent Yes a b a + b No c d c + d Total a +c b + d a+b+c+d How will find the strength of association between the disease under investigation and the suspected risk factor?
- 4. Odds Ratio (OR) } Odds ratio is the strength of association between the risk factor and outcome. } Assumptions: } The disease being investigated must be relatively rare } Cases must be representative of those with the disease } Controls must be representative of those without the disease } OR = } OR = ÷ = 6-Mar-19Epidemiological Exercises4 Odds of exposure among exposed (cases) Odds of exposure among non – exposed (controls) a b c d ad bc
- 6. Problem 1 6-Mar-19Epidemiological Exercises6 A study was carried out to find if there is a causal association between taking oral contraceptive pills (OCP) and developing cervical cancer. 80 women with cervical cancer and 120 women without cervical cancer were enquired about history of OCP use. 30 of the former group and 20 of the latter group and gave a positive history of OCP usage. 1. What type of study design is this? Justify your answer. (1+1) 2. Construct a 2x2 table for the above data (3 marks) 3. Calculate the appropriate measure of strength of association. (3 marks) 4. What is your interpretation of the results? (2 marks)
- 7. Problem 1 1. It is a case-control study, as cases and controls were selected and details of history of exposure were enquired. 2. Table: Case-Control (2 ✕ 2 Table) 6-Mar-19Epidemiological Exercises7 Intake of OCP Cervical cancer Total Present Absent Yes 30 (a) 20 (b) 50 No 50 (c) 100 (d) 150 Total 80 120 200
- 8. Problem 1 3. Odd’s ratio = ad / bc = 30 × 100 / 50 × 20 = 3 Yes, OCP usage is associated with increased development of cervical cancer } Odds of cervical cancer was 3 times higher among OCP users compared with controls. We can interpret this in a simpler way if the cervical cancer is rare and controls are representative of the same population from where the cases are taken; we can say that the risk of getting cervical cancer in OCP users is 3 times higher as compared with OCP nonusers. 6-Mar-19Epidemiological Exercises8
- 9. Problem 2 A study was conducted to find out if there was an association between cigarette smoking and developing lung cancer. 55 persons with lung cancer and 130 matched persons without lung cancer were enquired about history of cigarette smoking (> 5 cigarettes per day). 4 persons in former group and 45 persons in latter group gave a negative history. 1. What type of study design is this? Justify your answer. (1+1) 2. Construct a 2x2 table for the above data (3 marks) 3. Calculate the appropriate measure of strength of association. (3 marks) 4. What is your interpretation of the results? (2 marks) 6-Mar-19Epidemiological Exercises9
- 10. Problem 2 1. It is a case-control study, as cases and controls were selected and details of history of exposure were enquired. 2. Table: Case-Control (2 ✕ 2 Table) 6-Mar-19Epidemiological Exercises10 Smoker Lung cancer Total Present Absent Yes 51 (a) 85 (b) 136 No 4 (c) 45 (d) 49 Total 55 130 185
- 11. Problem 2 3. Odd’s ratio = ad / bc = 51 × 45 / 4 × 85 = 6.75 Yes, Smoking is associated with increased development of lung cancer } Odds of smoking was 6.75 times among lung cancer cases compared with controls. We can interpret this in a simpler way if the lung cancer is rare and controls are representative of the same population from where the cases are taken; we can say that the risk of getting lung cancer in smokers is 6.75 times as compared with non-smokers. 6-Mar-19Epidemiological Exercises11
- 12. Problem 3 A study was conducted to find the association between alcohol and development of oesophageal cancer. In total, 300 cases of oesophageal cancer and 280 healthy persons were enquired about the history of alcohol intake. 130 persons in former group and 40 persons in latter group gave a positive history of alcoholintake. 1. What type of study design is this? Justify your answer. (1+1) 2. Construct a 2x2 table for the above data (3 marks) 3. Calculate the appropriate measure of strength of association. (3 marks) 4. What is your interpretation of the results? (2 marks) 6-Mar-1912 Epidemiological Exercises
- 13. Problem 3 6-Mar-19Epidemiological Exercises13 1. It is a case-control study, as cases and controls were selected and details of history of exposure were enquired. 2. Table: Case-Control (2 ✕ 2 Table) Blindness Diabetes Total Present Absent Present 130 (a) 40 (b) 170 Absent 170 (c) 240 (d) 410 Total 300 280 580
- 14. Problem 3 3. Odd’s ratio = ad / bc = 130 × 240 / 40 × 170 = 4.6 4. Odds of alcohol intake was 4.6 times among oesophageal cancer cases compared with controls. We can interpret this in a simpler way if the oesophageal cancer is rare and controls are representative of the same population from where the cases are taken; we can say that the risk of getting oesophageal cancer in alcohol takers is 4.6 times as compared with alcoholnon-takers. 6-Mar-19Epidemiological Exercises14