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Question 1
In a case-control study of oral contraceptives and myocardial infarction (heart attack), exposure to birth control pills was abstracted from medical records at the time of the myocardial infarction. Results might be biased toward finding an association by all the following except:
If exposure to oral contraceptives was recorded just after the myocardial infarction, it could not have been a cause of it. All of the other choices could have artificially increased exposure in cases, resulting in a falsely elevated odds ratio.
Question 2
Investigators in Europe did a case-control study, nested in a multi-country cohort of more than 520,000 participants of vitamin D concentration and the risk of colon cancer. They studied 1,248 cases of incident colon cancer arising in the cohort and an equal number of controls, sampled from the same cohort and matched by age, sex, and study center. Vitamin D was measured in blood samples taken years before diagnosis. Vitamin D levels were lower in patients with colon cancer, independent of a rich array of potentially confounding variables. The study results could be described by any of the following except:
Even an exemplary case-control study such as this one should not claim that it has identified a cause because unmeasured confounding is always possible.
Question 3
Which of the following is the most direct result of a case-control study?
Case-control studies produce only odds ratios, which can be used to estimate relative risk.
Question 4
The epidemic curve for an acute infectious disease describes:
Epidemic curves describe the rise and fall in the number of cases over the time of the outbreak.
Question 5
Which of the following is the best reason for doing a case-control analysis of a cohort study?
One can always obtain a crude relative risk from a cohort study. However, if the cohort data do not contain all of the variables that should be controlled for, a case-control analysis of the cohort study is a more efficient approach to including the additional data because the data needs to be collected only for cases and controls, not for the entire cohort.
Question 6
The best way to identify cases is to obtain them from:
It would be better to sample cases and controls from a cohort rather than a dynamic population, especially if exposure or disease is changing rapidly over time and if controls are not matched to the date of disease onset for cases.
Question 7
What is the best reason to include multiple control groups in a case-control study?
Multiple control groups (not to be confused with multiple controls per case) are a way of examining whether the results are “sensitive” to the types of controls chosen, or whether the results using the different control groups are substantially different, which call the results into question.
Question 8
A case-control study was conducted based on a registry of 10,000 children that were genetically predisposed to develop celiac disease (characterized by sensitivity to gluten in the diet that can lead to malabsorption). Investigators wanted to see if enterovirus infection was associated with increased risk of developing the disease in these children. The children were followed with regular blood and stool tests for 10 years. After 10 years, a sample of 100 cases was selected from 2,000 children who developed the disease, and 20% of these children had a preceding enterovirus infection. 200 Controls were selected from the remaining children without the disease, and 13% of these children had enterovirus diagnosed. Which of the following is true?
All controls come from noncases in a case-control study, but sampling approaches vary. When noncases are selected using population-based methods throughout the study, the odds ratio is a direct estimate of relative risk. Noncases in this example are selected after study completion. When controls are selected in this way, the odds ratio will closely approximate the relative risk only if the disease is relatively rare (less than 1% to 5%). As outcomes become more frequent, the odds ratio tends to overestimate relative risk when >1 (as in this case) and underestimate relative risk when <1.
Question 9
Case-control studies can be used to study all the following except:
Case-control studies do not provide information on incidence (although if they are nested in a cohort, a cohort analysis of the same data can).
Question 10
In a case-control study of exercise and sudden cardiac death, matching would be useful:
Matching is used to control variables that might be strongly related to exposure or disease, to be sure that at least the cases and controls do not differ on those variables.
Question 11
In a case-control study of whether prolonged air travel is a risk factor for venous thromboembolism, 60 out of 100 cases and 40 out of 100 controls had prolonged air travel. What was the crude odds ratio from this study?
The crude odds ratio is obtained by creating a 2 × 2 table relating the number of cases versus controls to the number of exposed versus nonexposed people and dividing the cross products. In this case, the odds ratio is 60 × 60 divided by 40 × 40 = 2.25.
Question 12
A population-based case-control study would be especially useful for studying:
Case-control studies cannot study multiple outcomes because they begin with the presence or absence of only one disease, cannot report incidence, and cases should be incident (new onset), not prevalent.
Question 13
The prevalence odds ratio of rheumatoid arthritis provides an estimate of:
Odds ratios based on prevalent cases can provide a rough measure of association, but not a comparison of risk, which is about incident (new onset) cases.
Question 14
A case-control study was conducted to determine if children with asthma who used inhaled corticosteroids are at increased risk of fractures. A national registry was used to select cases and population-based controls, and possible confounders assessed by phone interview and chart review. Which of the following is incorrect?
Confounders may occur with different frequencies in cases and controls in the population under study. Confounding can be controlled in the design or analysis stages. Selecting controls matched on a confounder is a method of removing the effects of a confounder for that characteristic. Case-control studies use odds ratios to estimate relative risk but cannot estimate incidence. An association found in a case-control study can be one aspect of an argument for causation but cannot by itself establish causation.
Question 15
In an outbreak of acute gastroenteritis, a case-control study would be especially useful for identifying:
During the early phases of an outbreak, the offending microbe or toxin is usually known, but even if it is not, the most pressing question is how the disease is being spread. This information can then be used to stop the outbreak and identify the source.
Question 16
Sampling cases and controls from a defined population or cohort accomplishes which of the following?
If a case-control study is based on all or a random sample of cases and a random sample of controls from a population or cohort, the cases and controls should be like the population cases and controls (though cases and controls may differ in some characteristics, as with cohort studies).
Question 17
Case-control studies would be useful for answering all the following questions except:
Case-control studies do not provide information on incidence.
Question 18
In a case-control study of airplane flight and thrombophlebitis, all the following conditions should be met for the odds ratio to be a reasonable estimate of relative risk except:
The odds ratio approximates the relative risk when the disease is rare; a rule of thumb is <1/100. For more common diseases, the type of sampling should be population based for the odds ratio to approximate relative risk.