Simple Issue: Standard Deviation by Date range...

HelloPeople

I have a problem that would appear to be simple, but I am missing something:

A table listing of various websites (called siteIDs) has a range of dates and each date has associated figures (clicks and impressions).

What I would like to do is give a standard deviation calculation of impressions for each site ID, but each site ID has a random number of dates (rows) attributed to it- so no set formula.

I have a attached a sample set of data of 2 Site IDs, but bare in mind the full set is around 1000 rows so the manual SDEV formula I have used here would be too time consuming a process.

How do I set the row range for the STDEV always equal to the number of rows of the given SiteID.

Your thoughts,

Thanks,