Epidemiology module of practical skills for mbbs student


Evaluation of Study Designs



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Evaluation of Study Designs





Study 1

Study 2

Study 3

Study 4

Study 5

Study 6



Is this study design experimental?























Is it a cross sectional study?























Is this a prospective cohort study?























Is this a retrospective cohort study?























Is this a case-control study?


























Explain why you selected this study design:





Study 1

















































































Study 2


















































































Study 3


















































































Study 4


















































































Study 5


















































































Study 6









































































































Practical exercise

Case-control and Cohort Studies

A causal relationship between cigarette smoking and lung cancer was first suspected by clinicians in the 1920s on the basis of clinical observations. To test this apparent association Richard Doll and Austin Bradford Hill carried out two major studies. . The first was a case-control study, begun in 1947, in which the smoking habits of lung cancer patients were compared with the smoking habits of other patients. The second was a cohort study, begun in 1951, which related causes of death among doctors to their previously recorded smoking habits.


Part I: Case-control study

Data were obtained from patients in hospitals in and near London, over a 4-year period. Initially, 20 hospitals, and later more, were asked to notify the investigators of all patients admitted with a diagnosis of lung cancer. These patients and controls selected from in-patients with other disorders (primarily non-malignant) in the same hospitals at the same time were then interviewed about their smoking habits.

A total of 1,465 cases of lung cancer, all under age 75, were included in the study. These patients were culled from a larger number of which about 15% were not interviewed because of death, discharge, severity of illness, or inability to speak English.

Diagnoses of lung cancer in nearly all cases were based on biopsy, autopsy, sputum cytology wih or without bronchoscopic or radiographic evidence.



Interviews were conducted by four full-time social workers. Each worker, after interviewing a patient with lung cancer, interviewed a control patient of the same gender and the same 5-year age group. Table 1 compares cases and controls in terms of age and gender.

Table 1. Age and gender of cases and controls







Cases

Controls

Age group

Men

Women

Men

Women

25-34

17

3

17

3

35-44

116

15

116

15

45-54

493

38

493

38

55-65

545

34

545

34

65-84

186

18

186

18

Total

1,357

108

1,357

108



Table 2. Number and per cent of male cases and controls by amount smoked




Number of daily cigarettes

Cases

Controls




Number

%

Number

%


0

7

0,5

61

4,5

1-4

49

3,6

91

6,7

5-14

516

38,0

615

45,3

15-24

445

32,8

408

30,1

25-49

299

22,0

162

11,9

50+

41

3,0

20

1,5

Total

1,357

99,9

1,357

100,0


All smokers

1,350

99,4

1,296

95,5

The study showed a clear association between smoking and lung cancer(Table 2), but differences between case and control patients, particularly in terms of questionnaire response, might make the association spurious. Did lung cancer patients, knowing they had lung cancer, tend to exaggerate their smoking habits? Did controls, knowing smoking is ‘bad for you’ tend to under-report their smoking? Did interviewers, through prior knowledge of the hypothesis tend to exaggerate the smoking habits of lung cancer patients and minimize the smoking of controls? Possible answers to these questions came from patients initially interviewed as presumed lung cancer cases but later found not to have the disease. Table 3 compares the smoking habits of men aged 45-74 in this and other diagnostic groups.


Table 3. Percentage distribution of daily smoking habits for selected disease groups (standardized to age distribution of population of England and Wales)


Average daily number of cigarettes

Disease group

0

1-4

5-14

15-24

25+

Number of patients

Incorrectly presumed to have cancer

5,3

9,9

35,5

37,8

11,4



202

Confirmed lung cancer

0,3

4,6

35,9

35,0

24,3

1,224

Other lung diseases

1,9

9,9

38,3

38,7

11,2

301

Other cancers

4,6

9,4

47,2

26,0

12,8

473

Other diseases

5,6

9,0

44,8

26,9

13,7

875



Questions re Case-control Study


  1. How representative of all persons with lung cancer are cases admitted to hospital likely to be?




  1. How representative of all persons without lung cancer are other patients in hospital likely to be?




  1. What biases may have been introduced by failure to interview 15% of the cases?




  1. Why did the investigators require that control patients be of the same gender and age group as case patients?




  1. What do the data of table 2 suggest about the relationship between cigarette smoking and lung cancer?




  1. Compute the odds ratio for lung cancer occurring in all smokers compared with non-smokers. What does this represent? What does it suggest with regard to the etiological role of cigarette smoking in lung cancer?




  1. If the ‘control’ patients were asthma patients, would this change your opinion about the etiological role of cigarette smoking in lung cancer based on the results of this study?




  1. What does table 3 tell us? Why was age standardization of the data required?




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