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# Epidemiology Terms

### Basic terms and formulas of epidemiology, CU Denver, PH

QuestionAnswer
Proportion of newly developed cases Incidence
Measure of the frequency of an existing outcome at a point in time Point prevalence
Measure of the frequency of an existing outcome during a given period Period prevalence
Measure of the cumulative frequency of an outcome up to the present time Lifetime prevalence
General formula for incidence number of events in a defined population during a specified time period / population at risk
CI For a fixed cohort: CI = td0/I0 d= # of new events from time 0 to time t. I0 = # of individuals at risk for an event at time 0.
CI For a fixed cohort with censoring CI = td0/(I0-0.5(tc0)) c= # of censored subjects from time 0 to time t. Assumes censoring occurs uniformly from time 0 to t and thus are at risk for ½ the 0 to t time interval.
Range of CI 0-1
As time interval increases, CI approaches 1
Assumptions of CI (4) 1) A fixed (closed) cohort. There are some modifications of this assumption that require other assumptions (eg. censoring). 2) Uniformity of events within specified time interval. 3) Independence between censoring and events. 4) Lack of secular trends.
Units of CI No units
Cumulative survival : The cumulative probability of surviving an event over a specified period of time. tq0 = 1 – CI
Incidence Density The instantaneous rate of developing disease (having an event) in a population at risk. Also called incidence rate.
Formula for ID ID = td0/t(person-time)0
Range of Incidence Density 0 to infinity
Units of incidence density Incidence density is a rate and thus has units of 1/time.
Assumptions of Incidence Density 1) Risk of an event is constant over the observed time interval. 2) Independence between censoring and events. 3) Lack of secular trends. 4) Does not require a fixed (closed) cohort.
Incidence Rate based on aggregate data: Incidence rate = td0/((population0 + populationt)/2) x time)
Assumptions of Incidence Rate based on aggregate Data 1) See incidence density above. 2) Population and demographics relatively stable throughout the time period.
Risk probability of an indidiual developing a given disease over a specified period, conditional on not dying from any other causes during the period.
Created by: 597856662