Epidemiology MCQs

Epidemiology & Biostatistics Mock Examination Lent 2010

1. In a hypothetical clinical trial, a new drug was compared with "standard therapy" treatment. The endpoint was myocardial infarction. Which of the following best describes the primary reason to randomize patients to treatments? (Choose one best answer) a. to create two treatment groups that are similar at baseline on both known and unknown factors associated with myocardial infarction. b. prevent bias introduced when the patients know what type of treatment they are receiving c. prevent bias introduced when the investigators know what type of treatment the patients are receiving d. b and c

2. A randomized trial studied 242 HIV-seropositive, 2nd-trimester pregnant women to assess the efficacy of zidovudine (AZT) in preventing perinatal HIV transmission. Results were: Results from a randomized trial of the efficacy of zidovudine in preventing perinatal HIV transmission

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2a. Which one answer best describes the transmission rate in the table? a. b. c. d.

proportion relative rate absolute rate odds

b. Using the data in the table, estimate the relative risk of HIV infection for infants whose mothers took zidovudine relative to infants of mothers who took placebo. Show formula and calculations. 3. In a case-control study to investigate oral contraception and breast cancer (Choose one best answer) Options: A. cases would be women with breast cancer. B. controls would be given oral contraception. C. cases would be given oral contraception. D. women with breast cancer would be randomly allocated to be cases or controls. E. controls would be observed for several years to see how many developed breast cancer.

4. The paired t test is(Choose two best answers): A. impractical for large samples. B. equivalent to a chi-squared test. C. suitable for very small samples. D. used for independent samples. E. requires the assumption that differences between paired observations follow a Normal Distribution.

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5. For a t test for two independent samples to be (Choose two best answers): A. the numbers of observations must be approximately the same in the two groups. B. the standard deviations of observations must be approximately the same in the two groups. C. the means must be approximately the same in the two groups. D. the observations must be from distributions which are approximately Normal. E. the sizes of samples the must be small. 6. The standard chi-squared test for a 2x2 contingency table is not valid unless (Choose 1 best answer): A. all the expected frequencies are greater than five. B. both variables are continuous. C. at least one variable is from a Normal Distribution. D. all the observed frequencies are greater than five. E. the sample size is at least 100.

7. Regarding observational studies (Please answer true/false): a. b. c. d. e. f.

Produce definitive inferential evidence Can demonstrate causation are more useful for generating etiological hypotheses than testing or establishing them a difference in a study could be due to bias, confounding or chance Selection bias occurs in case control studies but not in cross-sectional prevalent studies. Selection bias is a distortion of study results that occurs because of the way subjects are selected into the study Loss to follow-up and subjects’ withdrawal from participation in the study, are the primary source of information or misclassification bias in cohort studies. g. Cross-sectional studies are better suited to generating hypothesis about exposure-disease relationship than to testing such relationship 8. Bradford Hill considerations for causal criteria can be summarised as (Please answer true/false): a. b. c. d. e. f.

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Temporality Dose-response relationship Consistency Plausibility Specificity Experimental evidence

9. 1319 children were questioned on prevalence of symptoms of severe cold at age 12 and again at 14 years. At age 12, 356 (27%) children reported to have severe colds in the past 12 months compared to 468 (35.5%) at age 14. (a) What kind of study is this (Choose one best answer)? a. b. c. d. e.

Case-Control Ecological study Cohort study Cross-sectional study Nested-case control

(b) To compare the prevalence of severe cold you should use (Choose one best answer): a. b. c. d.

the paired t-test X2 test McNemar test Fischer-Exact test

10. Please answer true or false: a. b. c. d. e. f. g. h. i. j. k. l.

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the larger the study the more power it has to detect a given difference. its best practice to write NS for a non-significant statistical test. it is best practice to do multiple statistical tests to find relationships in a data set. Power calculations are typically done after a study is completed. The statistical test to use when comparing 2 independent means (normal distribution) is the Cochran Q-test. a confounding variable is not associated with the outcome a confounding factor always is on the causal pathway Specificity= TN/TN+FP PPV= TP/TP+FN Criteria for screening are that the disease should be detectable at a latent stage Assessment of a screening program is done by whether more disease is diagnosed and by survival time. to assess outcome in a screening programme it is best to compare screened vs non-screened to reduce selection bias