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A closed cohort begins with 800 disease-free adults. During one year, 40 develop the disease and all participants complete follow-up. What is the one-year incidence proportion?

Free sample questions and explanations

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1. A closed cohort begins with 800 disease-free adults. During one year, 40 develop the disease and all participants complete follow-up. What is the one-year incidence proportion?
  1. 2%
  2. 5%
  3. 20%
  4. 40%

Answer: B

Divide new cases by the initially at-risk population: 40 / 800 = 0.05, or 5%. The one-year period must accompany this risk estimate. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
2. On 1 June, 120 of the 3,000 residents in a town have asthma. What is the point prevalence on that date?
  1. 0.4%
  2. 2.5%
  3. 4%
  4. 12%

Answer: C

There are 120 existing cases among 3,000 residents: 120 / 3,000 = 4%. This describes existing disease on one date, not new diagnoses during a year. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
3. A surveillance cohort records 18 new cases over 900 person-years at risk. What is its incidence rate per 1,000 person-years?
  1. 2
  2. 18
  3. 50
  4. 20

Answer: D

18 / 900 multiplied by 1,000 equals 20 new cases per 1,000 person-years. Person-time is the denominator; this is not a percentage. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
4. At a picnic, 24 of 80 people who ate a particular salad become ill. What is the food-specific attack proportion among salad eaters?
  1. 30%
  2. 24%
  3. 3%
  4. 80%

Answer: A

Use the people who ate the salad as the denominator: 24 / 80 = 30%. This calculation alone does not establish that the salad caused the illness. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
5. A survey asks residents whether they currently have a diagnosed chronic condition. It does not collect dates of diagnosis. Which measure can these data directly estimate?
  1. Incidence rate
  2. Point prevalence
  3. Case-fatality proportion
  4. One-year incidence proportion

Answer: B

A snapshot of current cases estimates point prevalence. Calculating incidence needs information about new cases and an appropriate at-risk period. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
6. A cohort contains 1,000 people, including 100 who already have the disease at enrolment. Of the remaining people, 45 develop it over one year with complete follow-up. What is the incidence proportion?
  1. 4.5%
  2. 10%
  3. 5%
  4. 15%

Answer: C

Exclude the 100 existing cases from the population at risk of a first diagnosis. The calculation is 45 / 900 = 5%. Reference: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html

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