Chapter 6 Review Questions (Key)

�6.1 Background

  1. A prevalence count includes all cases (old and new). An incidence count includes only recent onsets (i.e., onsets that occurred during the period of observation).
  2. This does NOT necessarily mean it is riskier to live in New York. New York is a large city. The greater number of deaths may merely reflect its large population size.
  3. size
  4. A ratio is the combination of two numbers that show their relative size. It is one number divided by another.
  5. cohort
  6. A stationary population is a population of contstant size and age.
  7. No. A stationary population must be open to allow for the steady-state inflow and outflow of people. 
  8. The average age of a cohort increases over time.
  9. A cohort can stay constant in size or (more likely) shrink over time. 
  10. Yes. An open population can increase in size through a birth  rate that exceeds its death rate and through an immigration rate the exceeds its emigration rate.
  11. The person is considered part of the cohort until they expire.

�6.2 Incidence Proportion

  1. No. of disease onsets. 
  2. Size of cohort at risk 
  3. Because they do not have the potential to develop the disease.
  4. Average risk, risk, cumulative incidence.
  5. (a) The period length of observation (b) The age range and distribution of the group 
  6. 1 in 40
  7. Factors that shape misapprehensions about risks include: (a) fear (b) lack of control (c) media attention (d) the inability to deal with numerical information
  8. Being struck by lightening.
  9. This is an open population, so this is a one-year rate.

�6.3 Incidence Rate

  1. 1 person observed for a year; 2 people observed for half a year each; 4 people observed for a quarter of a year each; etc.
  2. 68 person-hours
  3. Numerator = no. of disease onsets; Denominator = amount of "person-time" in population
  4. Synonyms: incidence density, average hazard
  5. The units of a rate in an open population are per person-time (inverse time), for example, per person-year, denoted year-1.
  6. Life expectancy = 1 /  0.01333 year-1 = 75.0 years
  7. Rate = 1 / 80 years = 0.0125 year-1 = 12.5 per 1000 person-years.
  8. 0.01333 year-1 = 13.3 per  1000 person-years. (Move the decimal point over three spaces, one for each 0 is 1,000)
  9. Rate = (2 people) / (295 people � 1 year) = 0.00680 year-1 = 6.80 per 1000 person-years
  10. Rate = (4 people) / (295 people � 2 years) = 0.0068 year-1 = 6.80 per 1000 person-years
  11. When the disease is rare (risk < 5%) and the period of observation is one year.

�6.4 Prevalence

  1. prevalence count
  2. population size
  3. None; prevalences are proportions, and proportions are unit-free “dimensionless” numbers.
  4. It considers both new and old cases and involves no follow-up of individuals (also see Table 6.2, p. 135).
  5. Prevalence will increase over time.
  6. Matching: (a) = prevalence; (b) = risk; (c) = rate