I didn't want to ask this in the Calc forum as it is likely to engender lots of useful advice about the various "If" algorithms.

If you were working on climate statistics and wanted to determine how many rainy days there are likely to be in any given year for any give region, the maths is relatively simple. I imagine you'd just divide the number of days in the year by the number of wet ones. Ad much the same idea to get monthly analysis or weekly etc.

So what slants do you put on the maths to do analysis over a longer time period?

As far as I can tell you'd only end up with a ratio of one in 6 or 10 or 27 or whatever it turns out to be.

So how do you go about ruling out what happens with sunspots or phases of the moon? What sort of If functions could you utilise?