C1M2

• Frequency


  Measured in different ways, can have different denominators
• Incidence


  Number of transitions from well to ill, from uninjured to injured, or from alive to dead in the study population during the study period
• Incidence rate

  Different from Incident Cases
• Cumulative incidence


  The total number of cases of an epidemic disease reported over time
• Prevalence


  Number of persons with a disease/condition in a population at a given point in time when the study was conducted
• Point prevalence


  Synonymous to prevalence
• Prevalent case


  Different from Prevalence rate
• Period prevalence


  Point prevalence at the start of the interval + incidence during the interval
• Prevalence can increase due to increase in yearly number of new cases or increase in length of time that symptomatic patients survive
• Point


  Specific point in time
• Period


  Specific time interval, the sum of points during the interval
• Risk


  Proportion of those unaffected at the start of the study period but had a risk event during the study period
• Cohort subsets (levels) of population

  • Persons who died
  • Persons who fell ill
  • Number of those infected
  • Number of exposed persons
  • Number of susceptible persons
  • Total population
• Susceptible population

  The cohort for infectious diseases
• Risk event


  Refers to death, disease, or injury
• Rates


  The number of events that occur in a defined time period, divided by the average number of people at risk for the event during that period
• To properly estimate the risk, the rate should have the following: 1) Event in the numerator occurs only once per person during the study interval, 2) Proportion of population affected by the event is small (e.g., <5%), 3) Time interval is short</b>
• Case fatality ratio

  Proportion of clinically ill who died
• Pathogenicity of organism

  Proportion of those infected who are clinically ill
• Infectiousness of organism

  Proportion of exposed persons who become infected