I have a long list of data recorded at about 20 second intervals (sometimes
more, sometimes less). I want to filter the data so that I only retain data
every 3-5 minutes. If the data were recorded exactly on the even minutes, I
could just divide by 180 or 300, but it sometimes drifts off with no set
pattern. Please explain how I can filter this data to sample only every 3-5
minutes.