Homework #2: Measures of Disease

Homework #2: Measures of Disease - EPI 420: SESSION 4 USES...

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E PI 420: S ESSION 4 Epi 420: Session 4 (Measures of Disease Exercise) Page 1 of 12 revised 4/4/2008 USES OF RATES IN EPIDEMIOLOGY (please complete all questions) The primary purpose of this exercise is to illustrate some methods of measuring the occurrence and outcomes of disease in populations. These methods are commonly used in describing the effects of disease in the population (descriptive epidemiology) and in investigations to test a hypothesis about disease occurrence (analytic epidemiology). The importance of the calculations and appropriate use of rates will be demonstrated by a variety of examples chosen from infectious and non-infectious disease studies. For convenience, the definitions, applications and limitations of the principal rates used in epidemiology are summarized in this exercise. Types and Uses of Disease Rates Disease rates measure the frequency of illness within specific populations. Time and place must always be specified. Rates appear in the following form: x . k y where x = number of times an event has occurred during a specific interval of time. y = number of persons exposed to the risk of the event during the same time interval. k = some round number (100; 1,000; 10,000; 100,000; etc) or base, depending on the relative magnitude of x and y. Rates are usually expressed as either crude rates or specific rates. Crude rates are concerned with the total number of events occurring in a defined population during a specified interval of time. Specific rates are expressed as the number of events of a disease occurring in a specifically defined population of a given age, race, and sex during a specified interval of time. Examples: Crude rate = total no. myocardial infarctions (MI) x 1000 per year total population of U.S. males Specific rate = no. white male MI's aged 45-54 _ x 1000 per year no. white males in population age 45-54 Note that both rates above are expressed per 1000 per year. Incidence Rate Incidence is a measure of the number of NEW cases of a disease in a population during a specified period of time. Incidence rates measure the risk of developing a disease. Incidence rate = no. NEW cases of disease over specified time period x 100,000 population at risk during specified time period
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E PI 420: S ESSION 4 Epi 420: Session 4 (Measures of Disease Exercise) Page 2 of 12 revised 4/4/2008 Note that in the above example incidence is expressed per 100,000 for a specified time period. Only new cases (fatal as well as non-fatal) are included in the numerator . The denominator includes an estimate of the population at risk (i.e. persons without the disease) at the beginning of the time period during which incidence is to be measured. It is a fundamental principle of the incidence rate that all persons included in the denominator must be at risk of being included in the numerator (of becoming a "case"), and that all of those included in the numerator must also be included in the denominator. Question 1.
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This note was uploaded on 05/12/2008 for the course EPI 420 taught by Professor Goldberg during the Spring '08 term at University of Washington.

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Homework #2: Measures of Disease - EPI 420: SESSION 4 USES...

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