AN OVERVIEW OF MEASUREMENTS IN EPIDEMIOLOGY FORMULAE USED TO COMPUTE THE MEASUREMENTS The following formulae are based on this typical epi 2 x 2 table with standard notation: Outcome (Disease) Yes No Yes a b a + b No c d c + d Exposure a + c b + d Other notation used: Io = Incidence of outcome among the unexposed (baseline risk) Ie = Incidence of outcome among the exposed It = Incidence of outcome in the total population (exposed and unexposed) Pexp = Prevalence of exposure in the population Po = Prevalence of outcome among the unexposed Pe = Prevalence of outcome among the exposed RR = Relative Risk (could refer to a Risk Ratio or a Rate Ratio) PR = Prevalence Ratio OR = Odds Ratio AR = Attributable Risk PAR = Population Attributable Risk ARR = Absolute Risk Reduction RRR = Relative Risk Reduction NNT = Number Needed to Treat CIR = Cumulative Incidence Ratio IDR = Incidence Density Ratio PF = Prevented Fraction
