Measures of Disease Frequency

Lecture on 8/21/2025

ratio

dividing one quantity by another

proportion

type of ratio, where numerator is a subset of the denominator

rate

type of ratio, which measures how quickly something happens with respect to something

prevalence definition

the number of people with a disease in the population divided by the total number of people in the population

useful for assessing the burden of disease in a population

tells us how much but not why or how

a function of the incidence of disease and the duration of disease

equation for prevalence

(#of people with a disease / total # of people in the population) at a specific time

point prevalence definition

proportion of a population that has disease at a specific point in time

point in time can be a specific calendar time or a lifetime event (birth, death, ect)

period prevalence definition

proportion of the population that has the characteristic at any point during a given time period of interest

the past 12 months is a commonly used period

incidence definition

frequency of new cases during a span of time in people who are at risk

focus is on measuring the probability of developing disease during a span of time

when calculating a population who is at risk (to calculate incidence), who should not be included in that count?

people who already have the disease, people who cannot get the disease

risk defintion

aka cumlative risk or incidence proportion

the number of new cases of a disease that occur during a specific period of time divided by the number of persons at risk of developing the disease at the beginning of the time period

equation for risk

#of new cases of disease over a specific period of time / #of persons at risk of disease at the beginning of the study period

when is risk not valid?

when a large number of people leave your study

loss of follow up

when people leave your study for any reason

what is incidence rate trying to determine?

the number of new cases during a particular person time of follow-up

time at risk

the time during which each person is at risk of getting the disease

if a person has already contracted the illness/disease, they often are not at risk of getting it again

equation for incidence rate

#of new cases of disease / total time at risk persons followed or amount of person time at risk

when calculating the risk, you need to determine the number of people at risk- you can make an assumption that they are at risk for a disease until…

they have the disease event, they die, they are lost to follow-up, or the study ends

equation for odds

p / 1-p

what are the pros and cons of risk (aka cumulative risk aka incidence proportion)?

easy to calculate and understand, less accurate when its assumptions are not met, more useful for fixed population

what are the pros and cons of incidence rate

greater accuracy, person-time denominator is more difficult to calculate and understand, more useful in dynamic populations

mortality rate

cumulative incidence of death due to a specific disease

morbidity rate

cumulative incidence of a disease that did not result in death

attack rate

the proportion of exposed people who develop disease

it is not really a rate, it’s a special type of cumulative incidence

which measure of disease frequency best describes… the frequency of new cases of TB in Boston over a calendar year

A) prevalence

B) risk/cumulative risk

C) incidence rate

D) none of the above

B) risk/cumulative risk

which measure of disease frequency best describes… the number of previously healthy women who has a stroke during 40,000 person-years of follow up?

A) prevalence

B) risk/cumulative risk

C) incidence rate

D) none of the above

C) incidence rate

what will happen to prevalence of a disease if a new treatment increases the survival among those affected by the disease (but all else remains the same)?

A) prevalence will increase

B) prevalence will decrease

C) prevalence will stay the same

D) not enough information

A) prevalence will increase

what event adds jelly beans to the prevalence jelly bean pot?

incidence

what events removes jelly beans from the prevalence jelly bean pot?

deaths and cures