Lecture 3: Measures of Frequency

Measures of Frequency

Measures that characterize the occurrence of disease in a specified population

counts, ratios, proportions, rates

four classes of MOF

count

a simple measure of quantity

ex: the number of HIV diagnoses in a community health clinic in 2017

incidence

a measurement of the new cases of disease that occur during a specified period of time

reflects a transition from disease-free to diseased state

Cumulative incidence (risk) and incidence density/rate (rate)

prevalence

a measurement of the existing cases of disease at a particular moment in time (or over a period of time)

= # of new cases during follow-up / # disease-free subjects at start of follow-up

cumulative incidence / risk equation

the proportion of disease-free individuals who contract disease during a specified time period

estimate of individual risk during the follow-up period

= # of new cases of disease / # of person-time units of observation

incidence rate equation

a measure of how quickly cases of a disease of interest occur

person-time

the sum of the periods of time that all persons at risk contributed to a study

the total amount of time that participants contributed to the study

risk

the probability that an event will occurr

rate

the measure of how quickly something happens

= # of observed cases at time t / population size at time t

point prevalence equation

measures the frequency of disease at a given point in time

= # of prevalent cases at beginning of time period + # of incident cases that develop during the period / size of the population for this same time period

period prevalence equation