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I need to do a 12 month rolling crime rate. I have monthly crime counts and population counts every 3 months.

For a calendar year I would usually use a population from mid-year as the denominator and the crime rate for the year as the numerator and then standardise using, say, 100,000. (i.e. crime count / population * 100 000).

However, now I need to do a rolling 12 month rate. I'm unclear Which population count to use as the denominator. Each method I can think of biases the results in some way. As I see it my options for population denominators are: (a) An average pop of the 12 months (b) The middle pop count for the 12 months (c) The count at the end of the 12 months

Which option is most appropriate or is there a more appropriate method?

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    $\begingroup$ (c) is clearly wrong. (b) doesn't use all the information. So go for (a). But do all three and keep two on hand, just in case someone queries your method. $\endgroup$ – Nick Cox Nov 1 '13 at 0:30

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