POPM 3240 Exam Unit 4 (2 week unit)
Questions All Answered Correctly
2026/2027 Updated.
Prevalence - Answer refers to the number of existing cases of disease (at a given time) divided
by the number of persons in the population at that time
Prevalence is simply a proportion and not a measure of risk because it does not take units of
time into account when the disease/outcome occurred
Point prevalence vs period prevalence - Answer Point prevalence would be the number of
students in the course who are sick with the flu today over the total number of students
enrolled in the course on that day.
Period prevalence would be the number of students who reported being sick with the flu at any
point over the semester divided by the number of students ever enrolled in the course over the
semester.
T or F: Prevalence can measure disease risk - Answer False
Prevalence is simply a proportion and not a measure of risk because it does not take units of
time into account when the disease/outcome occurred
Incidence - Answer Incidence is a dynamic measure of disease or outcome occurrence over
time. To measure incidence requires at least two observations of the same individual: the first
to establish who is "at risk" or "susceptible" for the disease/outcome and the second to confirm
new cases of the disease/outcome.
Equation for the relationship between incidence and prevalence - Answer Prevalence (existing
cases)= Incidence(new cases) x duration of disease
T or F: Prevalence is greater than incidence for chronic disease - Answer True: For a chronic
disease or condition, prevalence will be greater than incidence because the duration is so long.
T or F: Incidence is greater than prevalence for short-term diseases/health outcomes - Answer
True: For a short-term illness which only lasts a brief period of time, prevalence will be lower
than incidence.
Incidence risk (cumulative incidence) - Answer Incidence risk is the proportion of unaffected
individuals (those who do not have the disease or outcome of interest) who, on average,
develop the disease/outcome over a specific time period.
, Equation for Incidence Risk - Answer IR = (# of new cases in a specified time period) / [initial
number at risk (NAR) - ½ (withdrawals)]
subtracting half the withdrawals assumes that individuals leaving the population did so (on
average) halfway through the time period.
Incidence Rate/incidence density - Answer Incidence rate is a true rate. It is the speed with
which new cases of disease/health outcome occur.
Calculation of incidence rate depends on whether you have individual-level follow-up data for
the period of time "at-risk" for the denominator.
Incidence rate calculation (exact denominator) - Answer IRR = (# of new cases in a population
during a specified time period) / (Net time individuals in a population are at-risk during time)
If you know the exact details of each individual's follow-up time, which is preferred because it is
more precise, then you are able to calculate incidence rate with an exact denominator
Incidence rate with approximate denominator (use when we only have summary data) - Answer
IRR = (# of new cases in a population during a specified time period) / [(NARinitial + NARfinal)/2]
* ITC)
In this formula, you average the number at-risk (NAR) from the start of the follow-up period
(NARinitial) to the end of the follow-up period (NARfinal) and multiple this value by the internal
time component (ITC) in order to produce the denominator.
What is "internal time component"/ITC? - Answer A "true" rate always has an internal time
component (ITC) in the denominator, which is equal to or less than the time between when you
took your initial and final measurements.
What does incidence rate measure? - Answer Incidence rate measures two things: 1) the
number of new cases among those at risk in a population, and 2) the number of new cases
within a specific time period. When combined, we get formulas such as the two highlighted
above. The denominator ends up in units of individual time (e.g., person-year, animal-month).
Crude risk - Answer Crude risk is the "number developing any disease (for morbidity) or dying
(mortality) within the specified period of time" divided by the "number at risk minus half the
withdrawals" from the study.
Crude rate - Answer Crude rate is the "number developing any disease (for morbidity) or dying
(mortality) within the specified period of time" divided by the "average number at risk
multiplied by the internal time component" of the study.
Questions All Answered Correctly
2026/2027 Updated.
Prevalence - Answer refers to the number of existing cases of disease (at a given time) divided
by the number of persons in the population at that time
Prevalence is simply a proportion and not a measure of risk because it does not take units of
time into account when the disease/outcome occurred
Point prevalence vs period prevalence - Answer Point prevalence would be the number of
students in the course who are sick with the flu today over the total number of students
enrolled in the course on that day.
Period prevalence would be the number of students who reported being sick with the flu at any
point over the semester divided by the number of students ever enrolled in the course over the
semester.
T or F: Prevalence can measure disease risk - Answer False
Prevalence is simply a proportion and not a measure of risk because it does not take units of
time into account when the disease/outcome occurred
Incidence - Answer Incidence is a dynamic measure of disease or outcome occurrence over
time. To measure incidence requires at least two observations of the same individual: the first
to establish who is "at risk" or "susceptible" for the disease/outcome and the second to confirm
new cases of the disease/outcome.
Equation for the relationship between incidence and prevalence - Answer Prevalence (existing
cases)= Incidence(new cases) x duration of disease
T or F: Prevalence is greater than incidence for chronic disease - Answer True: For a chronic
disease or condition, prevalence will be greater than incidence because the duration is so long.
T or F: Incidence is greater than prevalence for short-term diseases/health outcomes - Answer
True: For a short-term illness which only lasts a brief period of time, prevalence will be lower
than incidence.
Incidence risk (cumulative incidence) - Answer Incidence risk is the proportion of unaffected
individuals (those who do not have the disease or outcome of interest) who, on average,
develop the disease/outcome over a specific time period.
, Equation for Incidence Risk - Answer IR = (# of new cases in a specified time period) / [initial
number at risk (NAR) - ½ (withdrawals)]
subtracting half the withdrawals assumes that individuals leaving the population did so (on
average) halfway through the time period.
Incidence Rate/incidence density - Answer Incidence rate is a true rate. It is the speed with
which new cases of disease/health outcome occur.
Calculation of incidence rate depends on whether you have individual-level follow-up data for
the period of time "at-risk" for the denominator.
Incidence rate calculation (exact denominator) - Answer IRR = (# of new cases in a population
during a specified time period) / (Net time individuals in a population are at-risk during time)
If you know the exact details of each individual's follow-up time, which is preferred because it is
more precise, then you are able to calculate incidence rate with an exact denominator
Incidence rate with approximate denominator (use when we only have summary data) - Answer
IRR = (# of new cases in a population during a specified time period) / [(NARinitial + NARfinal)/2]
* ITC)
In this formula, you average the number at-risk (NAR) from the start of the follow-up period
(NARinitial) to the end of the follow-up period (NARfinal) and multiple this value by the internal
time component (ITC) in order to produce the denominator.
What is "internal time component"/ITC? - Answer A "true" rate always has an internal time
component (ITC) in the denominator, which is equal to or less than the time between when you
took your initial and final measurements.
What does incidence rate measure? - Answer Incidence rate measures two things: 1) the
number of new cases among those at risk in a population, and 2) the number of new cases
within a specific time period. When combined, we get formulas such as the two highlighted
above. The denominator ends up in units of individual time (e.g., person-year, animal-month).
Crude risk - Answer Crude risk is the "number developing any disease (for morbidity) or dying
(mortality) within the specified period of time" divided by the "number at risk minus half the
withdrawals" from the study.
Crude rate - Answer Crude rate is the "number developing any disease (for morbidity) or dying
(mortality) within the specified period of time" divided by the "average number at risk
multiplied by the internal time component" of the study.
