Comparing Rates 1940 120210 5 yr 1980 183810 5 yr crude 132710 5yr adjusted151

Comparing rates 1940 120210 5 yr 1980 183810 5 yr

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Comparing RatesHas the cancer mortality rate changed from 1940to 1980?Are people more likely to die in Floridathan in Alaska? 1940 120.2/10 5 /yr 1980 183.8/10 5 /yr (crude) 132.7/10 5 /yr (adjusted) 151 152 Vital Statistics of the US, 1991 153 Age adjusted Mortality Rates for FL and AK, 1988 Calculated by Direct Adjustment Florida = 1,996,000/(245.7 x 10 6 ) = 812.4 per 100,000 Alaska= 1,879,000/(245.7 x 10 6 ) = 764.8 per 100,000 154 Are people more likely to die in Florida than in Alaska? 155 Picking the ‘Standard’ Actual adjusted values will vary with the selection of the ‘standard’ population Comparisons between groups will usually remain fairly constant Important: All groups in the comparison should be adjusted to same standard 156
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27 Standard Populations 157 1979 1981 1983 1985 1987 1989 1991 1993 1995 Death Rate per 100,000 population 1200 1000 800 600 400 200 1940 standard Crude Death rate 2000 standard 158 Indirect Adjustment Standard population rates x observed population Apply age specific rates from standard population to age specific population under study Calculate ‘expected’ number of cases (if the rates from a standard population were applied) Then compare to what was observed: SMR = observed/expected x 100% 159 Indirect Standardization 160 SMR = o/e x 100% = 90.7% 89.9% * * * * * * * Interpreting the SMR < 100% This population has fewer events than you would expect based on the standard rates = 100% This population has the same number of events that you would expect based on the standard rates > 100% This population has more events than would be expected based on the standard rates 161 When to use indirect adjustment? When category specific rates are not known When populations are small (and rates not stable) Occupational settings Small communities for short time periods 162
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28 Direct vs Indirect Adjustment Adjustment Strata pop X Strata Rate Direct Adjustment Standard Population X OBSERVED RATES Indirect Adjustment OBSERVED POPULATION X Standard Rates 163 Summary Different types of rates Crude, category specific How to compare two rates? Strata specific always appropriate Direct adjustment for summary comparisons Direct: Standard populations Indirect: Standard rates 164 Interpretation: Chance Introduction 165 Learning Objectives Understand the role of chance in epidemiologic studies 166 Sampling Goal of study Determine the true relation between exposure and disease Actual results may vary Sample vs. whole population Sampling variability 167 Statistics 101 168 Urn holds 100 marbles red and/or blue draw 4
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29 Statistics 101 169 Red Blue Conclusion 0 4 All blue 1 3 75% blue 2 2 50% blue 3 1 25% blue 4 0 All red Statistics 101 170 Red Blue Probability Conclusion 0 4 6% All blue 1 3 25% 75% blue 2 2 38% 50% blue 3 1 25% 25% blue 4 0 6% All red 50 Blue, 50 Red Statistics 101 171 50 Blue, 50 Red Sample Size Probability of sample with 1 color (%) 4 12.0 5 5.6 6 2.7 7 1.2 8 0.6 9 0.3 10 0.1 15 0.0018 20 0.000018 Statistics 101 Given Hypothesis the chance of drawing a red marble on any one try is 50% information about sample size Possible to calculate probability of a bad sample, or observing a particular result from a set of sample data 172 Statistics 101 sample size sampling variability and probability of an unrepresentative sample 173 “Bad samples happen” – S. Pettygrove
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