Filter By Time
 

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.

Filter By Time
 

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.

Filter By Time
 

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.