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Measures of Central Tendency

 

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There are several means by which the occurrence of disease may be measured. The commonly used measures of incidence and prevalence can be distinguished by differences in the time of disease onset. Incidence is a count of new cases of the disease (or outcome). Prevalence,  on the other hand, counts both new and existing cases of the disease.

Typical outcomes for an epidemiologic study, (sometimes referred to as the ‘D’s of Epidemiology) are as follows:

Outcomes of Epidemiology:

  • Death
  • Disease/Illness –  Physical signs, laboratory abnormalities
  • Discomfort – Symptoms (e.g., pain, nausea, dyspnea, itching, tinnitis)
  • Disability – Impaired ability to do usual activities
  • Dissatisfaction – Emotional reaction (e.g., sadness, anger)
  • Destitution – Poverty, unemployment

The first two, death and disease are the most commonly used.

Measures of Disease Frequency

Epidemiologic measures of disease frequency are of 5 types:

  1. Count: the number of individuals who meet the case definition;
  2. Proportion: A/(A+B); a fraction in which the numerator (A) includes only individuals who meet the case definition and the denominator totals the numbers of individuals who meet the case definition plus those in the study population who do not meet the case definition and are at risk.
  3. Ratio: A/B; a special fraction in which the numerator includes only individuals who meeting one criterion (e.g. the case definition) and the denominator includes only individuals in the study population who meet another criterion (e.g. do not meet the case definition but are at risk). A ratio is not dependent upon time.A ratio as a measure of disease frequency is used infrequently, in special situations. (not to be confused with an odds-ratio or risk-ratio)
  4. Rate: a fraction in which the numerator includes only individuals who meet the case definition and the denominator includes individuals in the study population who do or do not meet the case definition but could meet the case definition (at-risk).A rate is dependent upon time. In other words, a proportion over a particular period of time. An epidemiologic rate will contain the following: disease frequency (numerator), unit of population size, and the time period during which the event occurred
  5. Risk: the probability of an individual meeting the case definition (person-time rate). Risk is dependent upon time.