LESSON 2: DISEASE FREQUENCY

• To calculate measures of disease frequency:

  • count of events
  • population size
  • Period of observation
  • constant, K
• Morbidity: presence of disease in a population
• Two categories of measurement to describe frequency of disease:

  • Incidence (new cases)
  • Prevalence (existing cases - old and new)
• Incidence: frequency of occurrences of disease, injury, illness, or death. Number of newly diagnosed cases.
• incidence can be measured by:

  • counts = incidence cases/incidence
  • element of time = incidence rate
  • Proportion = cumulative incidence
• Rates of the occurrence of births, deaths, and new diseases all are forms of an incidence rate.
• Incidence rate: A rate calculated as the number of incident cases over a defined study period, divided by the population at risk at the midpoint of that study period.
• Incidence rate: rate is a measurement of disease risk. transition from a non-diseased state to a diseased state.
• Incidence rate’s numerator reflects the number of new cases during the time period
• Two types of denominators for incidence rate:

  1. Persons at risk (defined time period) - closed population
  2. Person-time (not observed full time) - open/dynamic population
• Incidence rate for dynamic population is also known as incidence density
• Incidence density is especially useful when the event of interest (e.g., colds, otitis media, myocardial infarction) can occur in a person more than once during the period of study.
• Cumulative incidence:

  • aka incidence proportion or risk
  • Probability that healthy people will develop disease during a specified time
  • proportion of initially Susceptible individuals in a population who develop new case of a disease om a specified time period .
• CUMULATIVE INCIDENCE

  
• This is a type of cumulative incidence: What is the risk that an individual will develop the disease over a given time interval?
• This is a type of incidence rate: How quickly or frequently does a disease occur in a population?
• Attack rates:

  • AKA cumulative incidence / incidence proportion
  • Usually applied to outbreak settings
• 3 types of attack rates:

  1. Overall attack rate- Total number of new cases/total population
  2. Food-specific attack rate - number of persons who ate specific food item and became ill/ total number of persons who ate the food
  3. Secondary attack rate - number of persons exposed to the risk factor for developing a disease within the incubation period following exposure to the primary case
• Prevalence (prevalent cases)

  • Not a measure of risk
  • Usually expressed as a percentage (multiplier = 100)
  • Used to express burden of disease (new + old cases)
  • Number of persons in a defined population who have a specified disease or condition at a given point in time
• Types of prevalence:

  • Point prevalence - prevalence of a disease at a certain point in time

  • Period prevalence- “How many people have had the disease at any point during a certain time period?” time period- month single calendar year, 5-year period
• Have you ever had allergies? Cumulative Incidence

  Do you currently have an allergy? Point Prevalence

  Have you had an allergy during the last 2 years? Period Prevalence
• prevalence:

  • Used by health planners for determining workload and monitoring control programs for chronic conditions
  • If incidence data not available, can use prevalence to estimate importance of disease
• Incidence rates are important in the study of acute diseases (sudden onset), whereas prevalence rates are useful in the study of chronic diseases
• • Differences in socioeconomic factors affect incidence & prevalence

  • High prevalence does not automatically mean poor health care