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nanmean different outcomes?

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JamJan
JamJan on 15 Mar 2019
Commented: JamJan on 15 Mar 2019
Hi,
I'm doing some analysis on data that has the following time intervals, however when I calculate the mean on two different ways (just for sanity) I ran into the following problem.
I have two arrays:
A = [NaN 1,57031250000000 1,60156250000000 1,71093750000000 1,65234375000000 1,51367187500000 1,49804687500000 1,49414062500000 1,59179687500000 1,41015625000000];
nanmean(A) = 1.5603
B = [NaN 1,64648437500000 0,796875000000000 0,667968750000000 1,65820312500000 1,69140625000000 1,39062500000000 1,66015625000000 1,40039062500000 1,57226562500000 1,65429687500000];
nanmean(B) = 1.4139
Now I have put them behind each other in a [A B] way.
C = [A B];
nanmean(C) = 1.4832
However this mean is not the same as the mean of A and B done separately.
(1.5603 + 1.4139)/2 = 1.4871
How is this possible and why is it different? Is this because of the NaNs?

Accepted Answer

Alex Mcaulley
Alex Mcaulley on 15 Mar 2019
Your two arrays don't have the same number of non nan elements (10 vs 11), then the mean is
(1.5603*sum(~isnan(A)) + 1.4139*sum(~isnan(B)))/(sum(~isnan(A))+sum(~isnan(B))) = 1.4832
  1 Comment
JamJan
JamJan on 15 Mar 2019
Thank you, indeed very logical!

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