• fixed- membership permenant and defined by event
• dynamic- populatin is transient and defined by being in or out of state

• number of individuals affected by disease (numerator)
• size of source population (denominator)
• length of time the population was followed

• ratio- division of one number by another
  • ex: BMI
  • ex: fetal death ratio=#fetal deaths/# live births
• proportion- numerator is subset of denominator, often express as percentage
  • a/(a+b)
  • ex: prevalence, case fatality
  • usually expressed as percentage or range      (0-1)
  • dimensionless
• rate- time is an intrinsic part of denominator but term is most misused

• definition- number of existing cases of disease in a population at a point or during a period of time
• eq: number of existing cases/total population

equation for cummulative incidence

number of new cases of disease/person-time of observation in candidate population

THIS INTEGRATES TIME INTO DENOMINATOR (so it is the only true rate)

• you have followed the entire population for the entire follow up period
  • you cant follow everyone for entire time period
  • so length of follow up is not uniform for all

• if low incidence with high duration, prevalance high
• if incidence high with shot duration, prevalance low

# fetal deaths during a year/ # live births in the same year

fetal death = loss of fetus after 22 or more weeks of gestation

• relative measure- calculate ratio of two measures of disease frequency (measure in exposed group and measure in unexposed comparison group)
• absolute measure- calculate difference between two measures of disease frequency (measure in exposed group and measure in unexposed comparison group)