Chapter 6 Review Questions (Key)
�6.1 Background
- 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).
- 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.
- size
- A ratio is the combination of
two numbers that show their relative size. It is one number divided by
another.
- cohort
- A stationary population is a
population of contstant size and age.
- No. A stationary population
must be open to allow for the steady-state inflow and outflow of
people.
- The average age of a cohort
increases over time.
- A cohort can stay constant in
size or (more likely) shrink over time.
- 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.
- The person is considered part
of the cohort until they expire.
�6.2 Incidence Proportion
- No. of disease onsets.
- Size of cohort at risk
- Because they do not have the
potential to develop the disease.
- Average risk, risk,
cumulative incidence.
- (a) The period length of
observation (b) The age range and distribution of the group
- 1 in 40
- Factors that shape
misapprehensions about risks include: (a) fear (b) lack of control (c)
media attention (d) the inability to deal with numerical information
- Being struck by lightening.
- This is an open population,
so this is a one-year rate.
�6.3 Incidence Rate
- 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.
- 68 person-hours
- Numerator = no. of disease
onsets; Denominator = amount of "person-time" in population
- Synonyms: incidence density,
average hazard
- The units of a rate in an
open population are per person-time (inverse time), for example,
per person-year, denoted year-1.
- Life expectancy = 1 /
0.01333 year-1 = 75.0 years
- Rate = 1 / 80 years = 0.0125
year-1 = 12.5 per 1000 person-years.
- 0.01333 year-1 =
13.3 per 1000 person-years. (Move the
decimal point over three spaces, one for each 0 is 1,000)
- Rate = (2 people) / (295
people � 1 year) = 0.00680 year-1 = 6.80 per 1000 person-years
- Rate = (4 people) / (295
people � 2 years) = 0.0068 year-1 = 6.80 per 1000 person-years
- When the disease is rare
(risk < 5%) and the period of observation is one year.
�6.4 Prevalence
- prevalence count
- population size
- None; prevalences are
proportions, and proportions are unit-free “dimensionless”
numbers.
- It considers both new and old
cases and involves no follow-up of individuals (also see Table 6.2, p.
135).
- Prevalence will increase over
time.
- Matching: (a) = prevalence;
(b) = risk; (c) = rate