��Chapter 6 (Incidence and Prevalence) Building Concepts Case Counts Prevalence count = no. of cases at particular time Incidence count = no. of case onsets that accumulate over time Counts without context of population size seldom useful - convert count to rates or proportions Types of Populations Closed population ( cohort ) Open population ( dynamic population ) **Incidence Proportion** Synonyms: risk, cumulative incidence Numerator = onsets Denominator = no. @ risk Closed population only Excludes those not at risk **Incidence Rate** Synonyms: incidence density, person-time rate Components Numerator = onsets Denominator = person-time @ risk Person-Time @ Risk In a cohort, you can sum person-time (e.g., 52 people observed for a week each = 1 person-year) In an open population in steady-state, use: Person-time ~= (average pop. size) � (time of observation) The reason for this approximation is given on p. 131 Example of a rate in a cohort [This of line drawings showins "survival" of individuals over time] Example of a rate in an open population [Merely use of formula: onsets / (average pop. size) � (time of observation) Interpretation of rates Speed Inverse of wait time ("life expectancy") Approx equal to risk **Prevalence** Prevalence count / population size Dynamics of Prevalence Figure 6.9 (p. 185) Increases in incidence �� increase prevalence Increases in duration (decreased resolution rate) �� increase prevalence Use of Multiplier to Report Rates Reporting per "m" does not change the incidence or prevalence, but does makes it easier to understand