Reasons for changes in incidence rates or prevalence

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Changes in disease incidence rates or prevalence (which may be monitored as part of routine disease surveillance) are often cited as evidence of cause and effect.

Experienced epidemiologists know, however, that changes in the recorded rates of disease are usually artefactual, rather than due to changes in the underlying rates of disease. Schneider lists the following reasons for changes:[1]


  • Errors in numerator due to
  • Changes in the recognition of disease. Examples include:
  • Where there have been suggestions in the medical or lay media that, for example, "acute floppy paralysis" is caused by oral polio vaccine.[2] when a concern such as this is raised, doctors are more likely to report cases, and patients are more likely to report to doctors with compatible illnesses.
  • The increase in reported cases of autism in the late 20th century, which are thought most likely to be due to changes in recognition of the disease and in diagnostic labelling.
  • Changes in gastrointestinal infection rates due to changes in the guidance to laboratories on when to test for specific pathogenic organisms.
  • Changes in the rules and procedures for classification of causes of death
  • Changes in the classification code of causes of death
  • Changes in accuracy of reporting age at death
  • Errors in the denominator due to error in the enumeration of the population


  • Changes in age distribution of the population
  • Changes in survivorship
  • Changes in incidence of disease resulting from
  • Genetic factors
  • Environmental factors, including vaccination and health promotion.


  1. Dona Schneider. Principles of epidemiology: lecture 6.
  2. Indian Medical Association, Plan International (India). Report of the National Consultative Meeting on Hepatitis-B & The Polio Eradication Initiative. (14th May). New Delhi.