LESSON 2: DEFINITION OF TERMS

• The frequency of a disease, injury, or death can be measured in different ways, and it can be related to different denominators, depending on the purpose of the research and the avail- ability of data.
• Incidence is the frequency of occurrences of disease, injury, or death—that is, the number of transitions from well to ill, from uninjured to injured, or from alive to dead—in the study population during the time period of the study.
• In epidemiology, numerator is:

  • count of persons with specific trait or condition
  • count of persons using a program or intervention
  • Count of positive or negative events
• In epi, denominator is:

  • total population in an area over a defined period of time
  • reference population in which health events are drawn and measured
• Sources of numerator:

  1. Surveys, registries, telephone book
  2. researcher’s own counts or observations
  3. context of formal study
• Sources of denomintor:

  • cencus
  • population statistics
• Ratio: division of one quantity by another
• Proportion: ratio in which the numerator is contained in the denominator.
• Proportion: tells us what fraction of the population is affected
• Rate: ratio representing change over time