Types of study designs 1. A study of traffic safety showed that 61% of those involved in accidents last year had more than 10 years of driving experience, 21% had 6 to 10 years experience, and 17% had 1 to 5 years. Traffic experts concluded that experience seems to make drivers more complacent and careless. a) What study design? Justify your answer (1 mark) b) Is it justified the conclusion? Explain why yes or not (2 marks) c) Explain how this design can be improved (3 marks) 2. In a fictitious survey, children and parents were interviewed in order to identify risk and protective factors for asthma. The following table was obtained. PR* 95% CI Father smoking 1.74 0.95 – 2.89 Eating fruit and vegetables 0.75 0.50 – 0.99 Viral infection at early age 0.44 0.23 – 0.67 Parent with asthma or hay fever 1.08 0.30 – 1.89 Lives in a metropolitan area population >1 million 1.38 1.11 – 1.71 *PR= Prevalence Ratio In your own words, what are the main conclusions from this study? (3 marks) 3. A comparison of two different countries gave the following results for coronary heart disease deaths. Crude incidence rate: Country A: 130 per 100,000 person years Country B: 130 per 100,000 person years Age-standardised incidence rate: Country A: 110 per 100,000 person years Country B: 180 per 100,000 person years Explain if age is confounding the comparison between country A and B. Explain why yes or not (4 marks) 4. In a case-control study of arsenic exposure and lung cancer the following results were obtained: LUNG CANCER ARSENIC EXPOSURE CASES CONTROLS Exposed 200 180 Not exposed 60 420 TOTAL 260 600 Is lung cancer associated with arsenic exposure? Show your calculations. Explain the meaning of your results (5 marks) 5. To study whether childhood leukaemia is related to the mother having X-rays during pregnancy, investigators choose 100 children with leukaemia and 100 healthy children. The investigators then interviewed all the mothers asking them whether they had X-rays during the pregnancy with that child. Explain two common sources of bias from this study. Give at least one reason. (4 marks) 6. The following table describes an unusual episode that actually occurred. Socio-economic Class Adult males Adult females Children Total N Death rate % N Death rate % N Death rate % N Death rate % High 173 66.5 144 3.5 5 0.0 322 37.3 Middle 160 91.9 93 16.1 24 0.0 277 58.5 Low 454 87.9 179 45.3 76 71.1 709 75.3 Unknown 875 78.4 23 8.7 0 – 898 76.6 Total 1662 81.0 439 23.5 105 51.4 2206 68.2 a) Provide a detailed description of the features of this table (2 marks). b) Formulate a hypothesis concerning the aetiology of this episode. Explain why your hypothesis is supported by the data from the table (2 marks). 7. A cohort study to evaluate the effect of smoking on lung cancer (LC) produced the following results: LC No LC Total Smokers 178 1129 1307 Non-smokers 79 1262 1341 The distribution of these people by age (less or more than 40 years old) is shown below: Among older than 40-years old: LC No LC Total Smokers 168 880 1048 Non-smokers 34 177 211 Among younger than 40-years old: LC No LC Total Smokers 10 249 259 Non-smokers 45 1085 1130 Discuss if age is confounding the association between smoking and lung cancer in this study. (5 marks for the calculations, 4 marks for the discussion) 8. The following table contains fictitious data from a cohort study evaluating the association between plasma levels of cholesterol (CHO) and cardiovascular disease (CVD). CVD No CVD Total High CHO 1,500 88,500 90,000 Low CHO 1,500 8,500 10,000 Total 3,000 97,000 100,000 What percentage of cases would we be prevented if an interventional program were able to decrease the cholesterol levels in this population? (3 marks) 9. A cohort study was conducted to evaluate the association between cholesterol level and risk of developing myocardial infarction. The data is presented in the following table: Myocardial infarction Yes Not Total High Cholesterol Yes 157 110 267 Not 209 313 522 Total 366 423 789 The following conclusion was made “The risk of developing myocardial infarction in people exposed to high levels of cholesterol is 147% higher than in those not exposed” Explain if this conclusion is correct or not. (2 marks)

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