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Understanding Disease Frequency: Incidence Rates and Epidemiological Measures

This chapter discusses the primary measures of disease frequency in epidemiology, focusing on incidence rates, prevalence, and cumulative incidence. The incidence rate is defined as the number of new disease cases divided by the person-time during which the cases arose. The chapter highlights the importance of classifying cases accurately and measuring incident cases using cohort studies. Additionally, it examines the calculation of incidence rates, mortality rates from major diseases in the U.S., and age-adjusted rates, emphasizing the need for clear definitions and accurate data to address public health challenges and disease prevention.

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Understanding Disease Frequency: Incidence Rates and Epidemiological Measures

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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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