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Chapter 4: Measuring Disease Frequency: Incidence Rate

Chapter 4: Measuring Disease Frequency: Incidence Rate. Measures of Disease Frequency. The primary measures of disease frequency in epidemiology are the incidence rate, prevalence, and cumulative incidence (risk).

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Chapter 4: Measuring Disease Frequency: Incidence Rate

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  1. Chapter 4:Measuring Disease Frequency: Incidence Rate

  2. Measures of Disease Frequency • The primary measures of disease frequency in epidemiology are the incidence rate, prevalence, and cumulative incidence (risk). • Measures of disease frequency quantify the occurrence of disease and injury in human populations.

  3. Definition of an Incidence Rate • The Incidence Rate (IR) is defined as the number of new cases of disease or injury divided by the amount of person/time during which the new cases arose. • The IR summarizes how fast people in a certain population are moving from one state of health to another state of health.

  4. Components of Incidence Rates • Numerator • Should not include prevalent (newly diagnosed) cases • International Classification of Diseases (ICD) Codes • Misclassification of cases • Mortality rates: Cases = Deaths

  5. Components of Incidence Rates continued • Denominator • Person-time, usually person-years • Person-time may be contributed only by people who are at risk for being a new case

  6. The Cohort Study • The type of epidemiologic study in which person/time and the number of new cases are measured is called a cohort study. • In a cohort study, two or more cohorts of people are followed over time, and their durations of follow-up and health experiences are recorded and then compared.

  7. Calculation of Incidence Rates Examples

  8. National Situation of Dengue and Dengue Hemorrhagic Fever in Thailand, 1958-2002

  9. Dengue/DHF by MonthThailand, 1997-2002

  10. Mortality Rates

  11. Leading Causes of Death in the U.S. • Diseases of the Heart • Cancers • Cerebrovascular Diseases • Chronic Lower Respiratory Disease • Injuries • Diabetes • Influenza and Pneumonia

  12. Age-Adjusted Rates • Age-Adjusted Incidence and Mortality Rates • Age-adjusted rates compared withage-specific rates • Age-adjusted rates compared with crude rates • Example: The Process of Age-Adjustment; Confounding

  13. Pictorial Representation of Confounding Age Distribution Living in Population ? ? ? Incidence A or B Rate

  14. Variations • Variation in Disease and Injury Frequency • Secular Trends • Morbidity and Mortality Worldwide

  15. Causes of Death • Causes of Death According to Exposure Status • Advantages in terms of disease and injury prevention • Leading causes of death by exposure status

  16. Obesity in the United States Source: www.cdc.gov/brfss/

  17. Epidemiology in Action: Disease Eradication • Smallpox • Malaria • Tuberculosis • Measles • Polio: Global Eradication Campaign

  18. Death from a Disease/Injury, Contributing to the Case Mortality Rate∑i and MR ∑i Injury IR ∑i Effects of ∑i IRs and MRs = New Cases of Disease and Injury People Free of Disease/Injury Disease IR ∑i Recover or Survive from a Disease/Injury Pictorial Representation of Incidence and Mortality Rates

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