Multiple Choice Questions for discussion Epidemiology Methods Multiple Choice Questions for discussion Taken from the Toronto Notes These MCQs do not all follow the current MCC guidelines (Oops ) March 2013

33) The purpose of randomization is to: a) make sure that there are equal numbers of men and women in test and control groups b) increase the chances of getting a statistically significant difference c) ensure that the numbers of cases and controls are equal d) limit bias e) all of the above d/D. Spasoff says A Randomization eliminates confounding by ensuring that the groups have a similar distribution of a wide range of prognostic factors. Sex could be one of those factors so (a) is ‘right’. However, sex/gender is often not a strong prognostic factor. Certainly, the ‘purpose’ of randomization is much broader than part (a). March 2013

34) Which of the following types of studies usually provides only a measure of prevalence? a) descriptive b) cross-sectional c) randomized controlled trial d) cohort e) none of the above (b/B). Cross-sectional studies focus on factors measured at a single point in time. That is the definition of prevalence. A strong argument could be made for (a) ‘descriptive’. However, the term ‘descriptive’ can be used to refer to various studies, some of which could produce incidence measures. Hence, (b) is a ‘better’ answer than (a). March 2013

35) A major advantage of cohort studies over case-control studies is that: a) they take less time and are less costly b) they can utilize a more representative population c) it is easier to obtain controls who are not exposed to the factor d) they permit estimation of risk of disease in those exposed to the factor e) they can be done on a “double-blind” basis (d/D). One of the main advantages to cohort studies is that they provide estimates of incidence (answer (d)). Part (a) is true but is not a big advantage in most cases. Part (c) is an inappropriate use of terminology: cohort studies don’t recruit controls; Spasoff says it depends how you use “control”; they may recruit [unexposed] comparison groups. March 2013

48) The incidence of a particular disease is greater in men than in women, but the prevalence shows no sex difference. The most probable explanation is that: a) the mortality rate is greater in women b) the case fatality rate is higher in women c) the duration of the disease is longer in women d) women receive less adequate medical care for the disease e) this diagnosis is more often missed in women Answer: (c/C). Prevalence = Incidence * duration (approximately and if the disease is ‘stable). Hence, if incidence is higher in men than women but prevalence is the same, then the duration of disease in women must be higher. This is likely due to a lower case fatality rate in women (the opposite of what option (b) says). March 2013

D This is the definition of Relative Risk March 2013

B. Based on the definition of AR. There could be confusion of the many definitions of ‘attributable risk’. However, none of the other options compute any of the other variants of AR. March 2013

12) The following indicate the results of screening test “Q” in screening for disease “Z”: The specificity of test “Q” would be: a) 40/70 b) 120/130 c) 40/50 d) 120/150 e) 40/200 B The main issue here is to make sure that the table is laid out the way you expect: on these exams, they sometimes switch the columns and rows (supposedly to make sure students are ‘thinking’ but, to me, it is unfair). Here, the table is fine. Just apply the definition. March 2013

The positive predictive value would be: a) 40/70 b) 120/130 c) 40/50 The main issue here is to make sure that the table is laid out the way you expect: on these exams, they sometimes switch the columns and rows (supposedly to make sure students are ‘thinking’ but, to me, it is unfair). Here, the table is fine. Just apply the definition. March 2013

a) a cross sectional/prevalence study 14) In which of the following study designs is the odds ratio the statistic typically used to show an association between cause and effect? a) a cross sectional/prevalence study b) a randomized controlled trial c) a cohort study d) a case study e) a case control study E March 2013

16) Alpha error is: a) the probability of declaring a difference to be absent when it in fact is present b) the probability of declaring a difference to be present when it is not c) the probability of declaring a difference to be absent when it is indeed absent d) the probability of declaring a difference to be present when it does exist e) none of the above B Application of the definition. March 2013

17) Which one of the following descriptors of a diagnostic test is influenced by the prevalence of the disease being tested for: a) specificity b) sensitivity c) accuracy d) positive predictive value e) reliability D PPV is strongly affected by prevalence: low prevalence leads to a marked drop in PPV. Accuracy is also affected by prevalence but this concept is not commonly used. Many people argue that sensitivity and specificity can be influenced by prevalence, especially if the composition of the ill and non-ill groups changes (diagnostic spectrum bias). March 2013

31) Which of the following statements does not apply to case control studies: a) starts with disease b) suitable for rare diseases c) relatively inexpensive d) prolonged follow-up required e) there may be a problem in selecting and matching controls D. Case-control studies require no follow-up of subjects. There can be a lengthy time to recruit cases. Also, many case-control studies are not cheap, especially if they involve DNA testing, etc. March 2013

32) A clinician who has been examining the patterns of mortality in your community says that the rates for heart disease and lung cancer are higher in this community than in an adjacent community. Which of the following questions should you ask first? a) how did the clinician choose the comparison community? b) have the rates been standardized for age? c) are tobacco sales significantly different in the two communities? d) are the facilities to treat these diseases comparable in the two areas? e) are the numbers of deaths comparable in each area? B. Nick did not like this question, but presumably it is getting at first ascertaining whether the observed difference is real. Hence the need to adjust for age. Only then would you pursue possible explanations, such as (c) or (d) since both CHD and lung cancer are smoking related diseases. Differences in smoking rates between the two cities could have a major impact on incidence and thus on mortality rates. Similarly, if one community had much worse CHD treatment, mortality could be elevated. This would be less important for lung cancer since treatment has little impact on mortality. March 2013

40) The effectiveness of a preventive measure is assessed in terms of: a) the effect in people to whom the measure is offered b) the effect in people who comply with the measure c) availability and the optimal use of resources d) the cost in dollars versus the benefits in improved health status e) all of the above A. Effectiveness asks the question: Does it work in people offered the treatment? (b) is efficacy (Can it work under optimal conditions?). (d) is cost-efficiency and (c) is a weird measure of societal cost preferences. March 2013

44) Of the five items listed below, the one which provides the strongest evidence for causality in an observed association between exposure and disease is: a) a large attributable risk b) a large relative risk c) a small p-value d) a positive result from a cohort study e) a case report B or D? This is another poor question. Nick would choose option (d), which implies that the outcome is statistically significant (low p-value) and of large enough size to ‘matter’. In addition, the cohort study address issue in temporality and permits direct estimation of risk. However, arguments can be made for (b) [a larger RR is more likely to be a true relationship] and (c) [less likely due to chance]. However, the question is not clear if the RR is adjusted (or if it could be confounded). It is clear that (e) is wrong and, in my view, (a) is addressing a different issue: impact rather than causation. March 2013

47) During a clinical trial, the difference in the success rates of two drugs was not statistically significant. This means that: a) there is no difference in drug effectiveness b) there is a sizeable probability that the demonstrated difference in the drugs’ effectiveness could occur due to chance alone c) the demonstrated difference in the drugs’ effectiveness is too small to be clinically meaningful d) the two samples of patients on which the drugs were tested came from the same population e) none of the above is true B. Option (b) really is describing the p-value (although it should have mentioned a null hypothesis). Some comment about power would also be appropriate but we would have to be given information about the sample size first. March 2013

49). Consider the following statements about statistical tests 49) Consider the following statements about statistical tests. Which statement is false: a) linear regression is used to describe the relationship between two continuous variables b) a confidence interval is a range of values giving information about the precision of an estimate c) ANOVA tables are used to make comparisons among the means of 3 or more groups simultaneously d) in a normal distribution, the mean, median and mode are equal e) the chi-square test evaluates the statistical significance of 2 or more percentages of categorical outcomes E. This is a poor question (but Ian modified it a bit to try & improve it). Nick discussed it with colleagues and could not identify the right answer (Ian thinks this is because they know way more about the topic than is expected from a medical student). Option (a) is technically incorrect because linear regression can have a non-continuous predictor variable (the Toronto Notes are wrong in this regard). Option (b) could be wrong if we pay overly close attention to the word ‘precision’. Option (c) is true (although ANOVA can be used with only two groups - another error in the Toronto Notes). Option (d) is true but technically doesn’t relate to statistical tests. Option (e) is probably true because the chi square would not be based on percentages, but is phrased in a very strange way. March 2013

More MCQs Here are some more questions that students can use to test their own knowledge: http://www.medicine.uottawa.ca/sim/data/Self-test_Qs_Epi_Methods_e.htm The questions contain comments on the answers, to illustrate why a given response is not correct March 2013