Measures of Disease Occurrence

Prevalence (P) Quantifies number of existing cases of disease in a population at a point or during a period of time
• P = Number of existing cases of disease / Number in total population (at a point or during a period of time)
• Ex. City A has 7000 people with arthritis on Jan 1st, 1999
• Population of City A = 70,000
• Prevalence of arthritis on Jan 1st = .10 or 10%
Incidence - Quantifies number of new cases of disease that develop in a population at risk during a specified time period
Three key concepts:
• New disease events, or for diseases that can occur more than once, usually first occurrence of disease
• Population at risk (candidate population) - can't have disease already, should have relevant organs
• Time must pass for a person to move from health to disease
Two Types of Incidence Measures
• Cumulative Incidence (Abbreviated CI): Number of new cases of disease / Number in candidate population over a specified period of time
• Used mainly for fixed populations because its assumes that everyone is followed for the entire time period
• Exp: Cumulative incidence of Sudden Infant Death Syndrome during first year of life: assumes you have followed all of the livebirths for one year or until SIDS occurred.
• Incidence Rate (Abbreviated IR): # new cases of disease /person-time of observation in candidate population
• Often you can't follow everyone for entire time period
• In a dynamic population, individuals enter population over time, become lost, etc.
• So length of follow-up is not uniform for all
• Incidence rates do not make assumption of complete follow-up
• This measure is a true rate because it directly integrates time into the denominator.

• Review of Dimensions
Prevalence = people/ people no dimension
Cumulative incidence = people/ people no dimension
Incidence rate = people/ people-time dimension is time –1

1. Reference: publichealth.jbpub.com/aschengrau/ppts/diseasefrequency.ppt
2. https://onlinecourses.science.psu.edu/stat507/02/occurence