Thank you! This got me the average difference between each year exactly like I asked. Is there any way to determine if the rates for allergen A, for example, are statistically significantly different between years? (e.g. is 2.5% different from 5.9% with a 95% CI and p-value?) In your code above, the output shows the average change between years (avga and avgb). Is there a way to see if these averages are statistically different from each other? Thanks again, Sherry
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