Lecture 04a - Comparing Rates and Proportions EPID 301...

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Comparing Rates and Comparing Rates and Proportions Proportions EPID 301 February 4, 2010
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Where Today’s Class Fits Where Today’s Class Fits Two weeks ago: Measures of disease frequency – incidence and prevalence Importance of denominator to account for differences in population size Today: Other factors such as age, sex etc. that can also affect comparisons of rates
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Crude vs Specific Rates Crude vs Specific Rates Crude Rate: Overall or summary rates for a defined population Specific Rate: Rates for distinct subgroups within a defined population Most commonly reported by age, sex or race/ethnicity Crude Rate = # of events in a given time period              # of individuals in defined population
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Comparing Rates – An Comparing Rates – An Example Example Crude mortality from cancer in the United States: 1940: 120.2 per 100,000 1980: 183.8 per 100,000 What reasons can you suggest for this difference?
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Comparing Rates – An Example Comparing Rates – An Example (2) (2) Age Distribution: United States 1940 and 1980
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Comparing Rates – An Example Comparing Rates – An Example (3) (3) Age-Specific Rates: What do you think now?
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Introduction to Confounding Introduction to Confounding Criteria for a confounder: Independently associated with exposure Independently associated with outcome Not on causal pathway Is age a confounder of this relationship? Population Membership (1940 vs 1980) Death from Cancer Age
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The Problem with Comparing Crude The Problem with Comparing Crude Rates Rates Crude rate is really a weighted average of the specific rates: Σ (category-specific rate x proportion of population in category) If 1) the distribution of the categories differs between the populations you are comparing, and 2) the rate of disease varies across the categories, The difference you see may be due to the variable that you categorized by and not the membership in one population or the other per se
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Specific Rates and Confounding Confounding Advantages:
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Lecture 04a - Comparing Rates and Proportions EPID 301...

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