Week 11 : Epidemiologic Data Measurements

• Frequency - refers to HOW a disease, injury, or death is being measured
• Prevalence - proportion of persons who have a condition DURING A PARTICULAR TIME
• Prevalence - includes both NEW and PRE EXISTING cases
• Incidence - transition from well to ill
• Incidence - limited to NEW CASES only
• Point Prevalence - the prevalence at a specific point in time (ONE TIME ONLY)
• Period Prevalence - the number of persons who had a given disease at any time during the SPECIFIED TIME INTERVAL
• Period Prevalence - is the sum of the point prevalence at the beginning of the interval plus the incidence during the interval
• TWO TYPES OF INCIDENCE

  • Incidence Proportion
  • Incidence Rate
• Incidence Proportion - probability of HAVING or DEVELOPING a disease
• Incidence Proportion - number of new cases of disease or injury during specified period/size of population at start of period
• Incidence Rate - person-time rate
• Incidence Rate - calculated for a long term cohort (group of people with the same characteristics)
• Incidence Rate - number of new cases of disease or injury during a specified period/time each person was observed, totaled for all persons
• Risk - persons unaffected at the beginning but experience a risk event during the study period
• Risk - proportion of initially disease-free population that develops a condition over a period of time
• Risk - numerator (new cases)
• Risk - denominator (number of persons in the population at the start of the observation period)
• Frequency Measures

  • ratios
  • proportions
  • rates
• Ratios - shows the relationship between 2 quantities
• Ratios - one interval or ratio scale variable to another
• Example of Ratios

  • sex ratio
  • hospital population
  • death to case ratio
• Proportions - special type of ratio
• Proportions - compare part to the whole
• Proportionate Mortality - specific type of epidemiologic proportio
• Proportionate Mortality - # of deaths from a specific cause/total deaths from all causes
• Rates - most common measure of disease frequency
• Rates - a special type of of ratio where numerator is related to the denominator and a measure of time is considered part of the denominator
• Rates - events in a defined time period
• Rates - useful for comparing disease frequency in different locations, at different times, or among different groups
• Rates - a measure of risk
• Rate cont - result by multiplying the resulting value to 1000
• Good Estime of Risk if

  • event in the numerator occurs only once
  • % of people affected is small - less than 5 %
  • time interval is short
• Rates - if the % of the population is larger or the time interval is longer, the RATE will be larger than the RISK
• Rates - if the event in the numerator will happen more than once, incidense density will be used instead of the rate
• Epidemic - means a disease or condition affecting a greater than expected number of inviduals within population, community, regions, at the same time
• Endemic - is when there is normal occurence of the disease in the population
• Pandemic - happens when the disease or conditions is geographically widespread
• In epidemics, rates are used in proportions with case counts/size of population
• Attack rate or Incidence proportion - proportion of population that develops illness during an outbreak