Questions in Epidemiology

1. Sensitivity refers to:
– The ability of a screening test to correctly identify those screened individuals who have the disease
– The ability of a screening test to identify no diseased individuals who actually do not have the disease
– The ability of a measuring instrument to give consistent results on repeated trials, regardless of its accuracy
– The proportion of those screened positive who actually have the disease

2. A screening examination was performed on 550 persons for Factor X, which is found in disease Y. A definitive diagnosis for disease Y had been obtained previously. The prevalence of the condition was 10% in the population and the results of the screening are shown in the table below. Please use the table for questions 2 through 6:
Disease Status
TEST RESULTS Disease Y Present Disease Y Absent
Positive for Factor X 40 50
Negative for Factor X 10 450
The sensitivity of this test is:
– 10%
– 44%
– 80%
– 90%
– 98%

3. The specificity of this test is:
– 10%
– 44%
– 80%
– 90%
– 98%

4. The positive predictive value of this test is:
– 10%
– 44%
– 80%
– 90%
– 98%

5. The negative predictive value of this test is:
– 10%
– 44%
– 80%
– 90%
– 98%

6. The number of false positives in this screened population is:
– 10
– 50
– 460
– 90

7. Had the prevalence of the condition Y in this population of 500 been 25% instead of 10% and the sensitivity and specificity of the screening test remained unchanged, what would have been observed with the positive predictive value?
– It would decrease
– It would increase
– It would remain unchanged
– It cannot be determined

8. Use of sequential screening in which only those who test positive to the first test are administered the second test generally results in:
– A gain in net specificity
– A gain in net sensitivity
– A decrease in net sensitivity
– An increase in both sensitivity and specificity

9. Lead time bias is:
– A bias that occurs because screening tends to identify cases with less aggressive forms of the disease
– An actually longer survival time for persons identified during a screening program because they were given an effective treatment
– An apparently lower survival rate among persons screened compared to an unscreened group
– An apparently longer survival time among persons identified during a screening program because they were identified at an earlier stage of their disease

10. Length-bias can be described as:
– A bias that occurs because screening tends to identify cases with less aggressive forms of the disease
– An actually longer survival time for persons identified during a screening program because they were given an effective treatment
– An apparently lower survival rate among persons screened compared to an unscreened group
– An apparently longer survival time among persons identified during a screening program because they were identified at an earlier stage of their disease

11. This form of bias may appear to indicate an increase in incidence of the condition that can be attributed to an increase in the number of false positives.
– Lead time bias
– Volunteer bias
– overdiagnosis bias
– A prognostic selection bias

12. A screening test for HIV had sensitivity of 99% and specificity of 90%. When used in a drug abuse clinic the test was found to have a positive predictive value of 85%. When used to screen a group of blood donors, the test was found to have a positive predictive value of 30%. Which of the following best explains this difference between the positive predictive values?
– Measurement error
– The prevalence of HIV is higher among those in the drug abuse clinic than among blood donors
– Cases of HIV are more severe among those in the drug abuse clinic
– Interobserver variation

13. Conditions favorable to population screening for presymptomatic diagnosis of a given disease include:
– A relatively high prevalence of the disease
– Availability of effective treatment
– A screening test with a high degree of sensitivity
– All of the above

14. Adverse consequences of using a screening test that has a low specificity include:
– Unnecessarily subjecting people to a potential risk associated with diagnostic procedures
– Possible psychological trauma that accompanies suspicion of a disease
– Both of the above
– Neither of the above

15. A new test has been developed to screen for a particular disease. Initially a test result of X or higher was considered positive for the disease. Researchers determined that the sensitivity of the test was unacceptably low. To increase the test’s sensitivity, it would be possible to:
– Lower the minimum test result below X
– Raise the minimum test result above X
– Use a population with a higher prevalence of the disease
– None of the above

16. What would the effect be on the specificity of the test if the sensitivity is increased?
– Specificity would remain the same
– Specificity would increase
– Specificity would decrease
– It is uncertain what the effect would be on the specificity with the information provided