# [rrd-users] Re: Calculation of 30min, 2hours and 1day average samples

Alex van den Bogaerdt alex at ergens.op.het.net
Thu Jun 5 11:11:37 MEST 2003

```On Thu, Jun 05, 2003 at 04:54:32PM +0800, Cody Lo wrote:

> I expect the 30min sample is the sum of the past six 5min samples and divide by 6.

No, not the last.

30-minute samples define the interval from hh:00 to hh:30 and hh:30 to (hh+1):00
so they must be built from specific 5-minute intervals, not semi-random intervals
such as "the past six".

And yes, averages can be averaged into a longer interval:

300*10 + 300*20 + 300*30 + 300*40 + 300*50 + 300*60
= 300*(10+20+30+40+50+60)
= 300*6  * ((10+20+30+40+50+60)/6)
= 1800 * 35

HTH
Alex
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