Lecture 2, part 4 Flashcards
What’s the denominator of prevalence?
Total population
What is a major use of prevalence?
Resource planning
Advantages of prevalence
Measures the overall burden of dz
Useful for chronic dzs which have long duration and difficult to pinpoint onset
Disadvantages of prevalence
Based on both incidence and duration of illness
Poor choice for dzs or outcomes with short duration or high mortality rate
Type of # of cumulative incidence
Proportion
Units of cumulative incidence
None
Range of cumulative incidence
0 to 1
Numerator of cumulative incidence
New cases
Denominator of cumulative incidence
Pop at risk
Major uses of cumulative incidence
Research on causes, prevention, and tx of dz
Advantages of cumulative incidence
Measures the risk or probability of developing the dz during the study period
Easy measure for ppl to understand
Disadvantages of cumulative incidence
Less accurate than true rate
Risk can change over time, so selection of appropriate time period is important
Works better for fixed pops
Type of number for incidence rate
True rate
Units of incidence rate
1/time or t-1
Range of incidence rate
0 to infinity
Numerator of incidence rate
New cases
Denominator of incidence rate
Person-time at risk
Major uses of incidence rate
Research on causes, prevention, and tx of dz
Advantages of incidence rate
Conveys the speed at which new cases occur
Person-time can accommodate persons joining/leaving the study
Good for transient, dynamic pops
Disadvantages of incidence rate
Assumes that probability or risk of dz during the study period is constant
Difficult to calculate (can use mid-year pop for denominator)
Difficult to understand
Prevalence formula in relation to incidence
Prevalence = incidence x duration of dz
P / (1-P) = IR x d
P = proportion of total pop with the dz
1- P is the proportion of the total pop without the dz
d = avg duration of dz
When dz occurrence is rare, simplifies to P = IR x d
When does prevalence increase?
New cases are added to the existing cases
When does prevalence decrease?
People are cured, recover, or die
Possible interpretations for changes in incidence
Change in case definition or dz detection
Selective migration or pop changes
Change in prevention/intervention strategy
Introduction of a new risk factor
Change in habits/behaviors
Change in dz virulence/transmissibility
Possible interpretations for changes in prevalence
Change in case definition or dz detection
Selective migration or pop changes
Change in prevention/intervention strategy
Change in the incidence of dz
Change in the duration of dz
