EPI 320 Lecture Notes - Lecture 4: Mortality Rate, Cumulative Incidence
Wednesday, 3.28
Review of Monday (refer to notes from 3.26)
• Pros and cons of simpler measures of frequency (absolute numbers, ratios, proportions)
• Person-time
• What it means to be at risk in epidemiology
• Prevalence & Incidence (measure of disease occurrence/risk)
Poll Ev Q: As many as 31% of US adults have chronic pain. A: prevalence
Cumulative Incidence
• How fast people transition from non-diseases to diseased over time
• When a group of people are all observed over the same amount of time
• Incidence becomes simply a proportion
• Ex: 10-year incidence of breast cancer amount 40-year-old women is 1.47%
• Technically, not a rate → risk or with the modifier cumulative
What is a closed population?
• Cumulative incidence assumes a closed population
• The (denominator) population does not change over the length of the study/observation
o No births, deaths, immigration/emigration, no loss to follow-up
• Everyone starts out at risk → only way out of the at-risk pool is to develop disease
• Ex: passengers on a flight, participations in a four-hour drug safety study
What is an open population?
• Real populations look a lot more like this
• The (denominator) population is not fixed throughout the study
• Everyone starts out at risk
• People contribute person-time as long as they are observed and at risk, or until: death,
loss of follow-up, out-migration/removal from study, development of outcome of interest
• People stop contributing to person-time b/c they are no longer at risk
How do we handle an open population?
• Using incidence rate:
• This is a true rate and speaks to the speed of developing disease
• Default meaning of incidence is incidence rate
For
midterm,
look at
different
studies of
closed vs.
open
studies
