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.

does the data record the time the data is recorded?
If it does, in what form? there are several ways to do what you want

"Amateur George" wrote:

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.

Assume your data looks like this:
Tact Vala Tsap Vals
0:00:24 1.074 0:00:20 1.074
0:00:48 1.171 0:04:20 2.105
0:01:13 1.242 0:08:20 3.092
0:01:37 1.326
0:02:01 1.437
0:02:25 1.510
0:02:50 1.620
0:03:14 1.765
0:03:38 1.880
0:04:02 1.957
0:04:27 2.105
0:04:51 2.222
0:05:15 2.302
0:05:39 2.416
0:06:04 2.515
0:06:28 2.600
0:06:52 2.706
0:07:16 2.810
0:07:40 2.928
0:08:05 3.018
0:08:29 3.092

Tact happens on the average every 20 seconds
with a maximum random error of plus or minus 5 seconds.
Vala are measurements taken at Tact time.
Tsap is the sample time, that is
exactly every 4 minutes from some starting time.
If 00:00:24 is located at A2, then enter this array formula at D2:
=INDEX(Vala,MATCH(MIN(ABS((Tsap 2:2)-Tact)),ABS(Tact-Tsap 2:2),0))
and copy it down.