County Comparisons was written in response to a letter from a reader who said she only wanted to know one thing: when did the fallout land in her back yard? And how did her county rate in comparison with all the other counties in the United States? It was a simple question, but not easy to answer. It was first necessary to rank the counties in terms of local fallout for 1951 through 1970. After determining when the fallout descended on the counties, it was necessary to rank the counties by dates. And there were over 400 dates in which fallout occurred. After months of number-crunching, the results finally appeared. Now, readers would be able to learn not only when the nuclear fallout came down–along with the quantity–but how their county ranked against all the other counties in the United States for fallout on that particular day.