Clarify "average per month" in Data are too coarse section

[onyxfish]

Technically the yearly total divided by 12 is the monthly average. Explain why this is a terrible idea.

Explanation from an email I wrote:

With regard to your specific question about "average per month," I think I do need to clarify that section of the guide. Of course the yearly total divided by 12 is technically the monthly average. My point was that if you have no idea of the variation from month to month creating an average can be extremely misleading. Without knowing the distribution of the data it's entirely possible the entire total happened in the first day of the year, which would make expressing it as a monthly average absolutely incorrect.