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Introduction
  • Overview
    • rates measure event frequencies in a defined population during a specific time period
    • rates are helpful in comparing event frequency in various places or times with varying population sizes
Mortality Rates
  • Crude mortality rate
    • describes the total number of deaths per years per 1,000 people
    • equation
      • number of deaths during a certain time interval/mid-interval population size
        • e.g., number of deaths in a year from January-January/population size on June 30th
  • Cause-specific mortality rate
    • describes the mortality rate caused by a certain disease or cause
      • e.g., aggressive malignancy
    • equation
      • number of deaths by a certain disease during a certain time interval/mid-interval population
Infectious Disease Rates
  • Attack rate
    • a type of incidence measure that is usually used in outbreak epidemiology
      • e.g., gastrointestinal infection in patients caused by contaminated food
    • equation
      • number of patients with the disease during a certain time interval/total population at risk during that interval
  • Case-fatality rate
    • equation
      • number of deaths by a certain disease during a certain time interval/total population affected by the disease during that interval
Reproductive Rates
  • Crude birth rate
    • equation
      • number of live births/total populations size
  • Maternal mortality rate
    • equation
      • number of maternal deaths during a certain time interval/number of live births during that interval
  • Neonatal mortality rate
    • equation
      • number of deaths of children < 28 days old during a certain time interval/number of live births during the same time interval
  • Infant mortality rate
    • equation
      • number of deaths among children < 1 year of age during a certain interval/number of live births during that interval
      • similarly, can calculate under 5 mortality rate as well 
Rate Standardization
  • Rate standardization involves adjusting rates to allow for a comparison of frequency in populations with differing characteristics that may confound the results
    • e.g., if disease X is more common in older people and population A has a higher proportion of older folks in their population, without adjustment to account for these age differences, it may appear that individuals in population A are at higher risk of a disease when in fact a population-level factor (age structure) is confounding the risk association
  • Age is most commonly adjusted for 
  • Two main methods of standardization
    • direct
      • calculate expected rates of deaths (or disease cases) in studied population if it had a standarized structure with regards to the variable to be adjusted for
        • compare to crude rate or other standardized rate
        • asks, "how would mortality rates compare in 2 populations if their age distributions were identical?"
      • first use a "standard" population structure
        • can be chosen arbitrarily
        • e.g., proportions of U.S. population in each age groups
      • multiply each strata of the population (e.g., age groupsby specific rates observed for those strata in each population under study to get the expected cases 
      • add up all strata of expected cases and divide the total expected cases by the standard population total to get standardized rates
    • indirect
      • calculate expected number of deaths (or disease cases) in studied populations using standard population rates
        • compare to number of actual deaths (standardized mortality ratio) or to expected number in comparison population
        • asks, "how many deaths would be expected in a study population if that population had the same mortality rates as a standard population?"
      • first choose a "standard population" with variable-specific rates (e.g., rates of death in various age groups)
      • multiply number of people in each variable group in the study population by the standardized rates to get the expected number of deaths 
      • add the expected number of deaths together and divide the total observed deaths in study population by the expected deaths
        • this is known as the standardized mortality ratio (SMR)
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