[…] This has been a challenging data set to work with. While it’s interesting and potentially insightful, we have to remember that the sampling rates are low in places and the proportions are small enough that they can be affected by coding errors and other behaviors. Furthermore, proportions don't behave quite like "regular" numbers, but the Central Limit Theorem and the large number of counties gives us some comfort in using a normal distribution. With cleaner, integer data, we could have used a Binomial distribution for the confidence curves, as in Rick Wicklin's post on funnel plots. …