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Statistical Measures

Morbidity Frequency Measures

Prevalence Rates

Example: Comparing Prevalence and Incidence
Two surveys were done of the same community 12 months apart. Of 5,000 people surveyed the first time, 25 had antibodies to histoplasmosis. Twelve months later, 35 had antibodies, including the original 25. We will calculate the prevalence at the second survey, and compare the prevalence with the 1-year incidence.

  1) Prevalence at the second survey:
    x = antibody positive at second survey = 35
    y = population = 5,000
    (x/y) x 10n = 35/5,000 x 1,000 = 7 per 1,000
     
  2) Incidence during the 12-month period:
    x = number of new positives during the 12-month period = 35 – 25 = 10
    y = population at risk = 5,000 – 25 (already infected) = 4,975
    (x/y) x 10n = 10/4,975 x 1,000 = 2 per 1,000
     

Prevalence is based on both incidence (risk) and duration of disease. High prevalence of a disease within a population may reflect high risk, or it may reflect prolonged survival without cure. Conversely, low prevalence may indicate low incidence, a rapidly fatal process, or rapid recovery.