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DESCRIPTIVE EPIDEMIOLOGY

DESCRIPTIVE EPIDEMIOLOGY. CRUDE, SPECIFIC, and ADJUSTED RATES. Descriptive Epidemiology. Includes descriptive statistics that provide information on disease patterns by various characteristics of person, place, and time. Descriptive statistics are useful for:.

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DESCRIPTIVE EPIDEMIOLOGY

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  1. DESCRIPTIVE EPIDEMIOLOGY CRUDE, SPECIFIC, and ADJUSTED RATES

  2. Descriptive Epidemiology Includes descriptive statistics that provide information on disease patterns by various characteristics of person, place, and time.

  3. Descriptive statistics are useful for: • Providing clues about disease causation and prevention that are usually investigated further in formal studies. • Assessing the health status of a population (e.g. Healthy People 2010) 3. Allocating resources efficiently and targeting populations for education or preventive programs

  4. A. Descriptive statistics Routinely collected data from many sources including data on mortality and natality from vital records, reportable diseases from surveillance programs, other diseases from national surveys. These sources are described in Chapter 5. Many are now available through the Internet.

  5. Disease rates categorized by person, place and time • Person: Who has the disease? male vs. females, young vs. old, black vs. white • Place: Where is the disease more or less common? Different scales of geography: regions of earth, countries, states, counties, cities, neighborhoods • Time: Is the disease rate changing over time? Different scales of time: decades to seasons to days

  6. Age is an important personal characteristic

  7. Place: The Geography of Breast Cancer

  8. Changes over time: Breast cancer incidence and mortality rates over the decades

  9. If morbidity or mortality from a given disease changes over time, you can infer: • Some cause of the disease must also be changing • Or there is an "artifactual" explanation For example, there are differences in disease definition, diagnosis, or reporting over time. Or there are changes in enumerating the population denominator of the rate

  10. Epidemiology in the NewsGains on Heart Disease Leave More Survivors and Questions (NY Times, 2003) • The numbers have been inching down for decades…They remain the leading cause of death in the U.S. but their toll is nothing like it used to be. • The plunging death rates have come about for an array of reasons –new drugs, new treatments, changes in behavior….Tens of millions of Americans are taking cholesterol lowering drugs, there is better control of hypertension.

  11. Epidemiology in the NewsGains on Heart Disease Leave More Survivors and Questions (NY Times, 2003) • The decline in smoking rates did not markedly affect heart disease death rates…Smokers were dying early of lung cancer and lung disease, taking them out of the pool of people who might die of cardiovascular diseases. • Data on death rates do not show whether fewer people are developing cardiovascular disease in the first place. • With better methods of detection, doctors are picking up heart attacks that would have gone unnoticed in previous years.

  12. C. Crude, Specific and Adjusted Rates 1. Crude rate: • A summary measure calculated by dividing the total number of cases in the population by the total number of individuals in that population at a specified time period

  13. C. Crude, Specific and Adjusted Rates 2. Category specific rates: • Rates specific to some particular sub-population: age-specific, race-specific, sex-specific

  14. C. Crude, Specific and Adjusted Rates 3. Problems comparing crude rates among populations: • Groups differ with respect to underlying characteristics that affect overall rate of disease (especially age, sex, and race) and so you may be making an unfair comparison

  15. Example: Overall Mortality Rates in Alaska and Florida in 1998

  16. Example: Overall Mortality Rates in Alaska and Florida • The difference in the age structure between the populations of Florida and Alaska make this an unfair comparison • You could get around the problem by comparing age-specific mortality rates between the two states but this is cumbersome.

  17. Age-Specific Mortality Rates in Alaska and Florida

  18. Age-adjusted rate • Summary rate that accounts for age difference between populations. • Any differences between rates cannot be attributed to age.

  19. Calculating crude rates Crude death rate in Florida can be calculated in two ways: 1. Total number of deaths / total population 158,167 / 14,908,230 = .0106094 = 1060.94 / 100,000

  20. 2. Or crude rate can be considered the weighted average of age-specific rates, with weights equal to the proportion of the population in each category.

  21. Thus, the crude death rate in Florida can be calculated as weighted average: (percent of population in that age group) x (age-specific rate) (.064) (181.40/100,000) + (.197) (36.78/100,000) + (.342) (178.23/100,000) + (.215)(725.04/100,000) + (.183) (4,517.68/100,000) = 1060.94 / 100,000 Note: Even if the two populations have identical age-specific rates, the crude rate will vary if the age distribution of the populations differ -- that is, if there are different proportions of people in each age category.

  22. Calculating crude rates

  23. Now let’s calculate age-adjusted rates • We want a summary number for both Alaska and Florida that allows for comparison with differences in age accounted for. These numbers are "adjusted" for age -- they are called age-adjusted or age-standardized rates. • They answer the question: what would the death rate be in each state if the population in each state had identical age distributions? • What age distribution? Any that you want! Usually the U.S. population in a census year is used.

  24. Data needed to construct age adjusted rates for Florida and Alaska Death rates per 100,000 1998 US population (% of total) Age groups Florida Alaska <5 181.40 149.19 18,989,257 (7.0%) 5-19 36.78 43.44 58,712,947 (21.7%) 20-44 178.23 165.97 100,919,429 (37.3%) 45-64 725.04 521.18 57,241,131 (21.2%) >65 4,517.68 4,011.94 34,385,239 (12.7%) Total 417.91 270,248,003 (100%) 1060.94

  25. Calculating age-adjusted rates • Age standardized rate: weighted average of age specific rates where the weights are the distribution of age in the standard population. This is called direct standardization.

  26. Age adjusted rate in Florida (.070) (181.40/100,000) + (.217) (36.78/100,000) + (.373) (178.23/100,000) + (.212) (725.04/100,000) + (.127) (4,517.68/100,000) = 815.15 / 100,000 Age adjusted rate in Alaska (.070) (149.19/100,000) + (.217) (43.44/100,000) + (.373) (165.97/100,000) + (.212) (521.18/100,000) + (.127) (4,011.94/100,000) = 702.56 / 100,000 Note that the weights are the same for Florida and Alaska

  27. These are hypothetical death rates that would have occurred in each state if each state had the age distribution of the entire U.S. population in 1988. • The remaining difference between the two adjusted rates is not due to age. • Adjusted rates are good only for comparison -- alone they are meaningless.

  28. Comparison of crude and age-adjusted rates • Crude rate in Florida: 1060.94/100,000 • Crude rate in Alaska: 417.91/100,000 • Age-adjusted rate in Florida: 815.15/100,000 • Age-adjusted rate in Alaska: 702.56/100,000 Was the crude comparison confounded by age? What is your conclusion about the difference in mortality rates?

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