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[ferret_users] ave vs. sum and ngd

Dear Users,
i know there is a difference between @ave and @sum, @ngd. It is not really a question, but still want to post it.

I list 3 timesteps:
yes? list/nohead temp[l=16:18]     
 16-MAR-1962 / 16:   1.74
 16-APR-1962 / 17:   8.74
 16-MAY-1962 / 18:  15.34

Here is the real average for it. The mean i guess.
yes? list/nohead temp[l=16:18@sum]/3

The (weighted) "average" is slightly different. Though if there is no missing value, they should be the same.
yes? list/nohead temp[l=16:18@ave]

Why is that weightening in @ave? The 3 timesteps are equivalent for me. I should just use the sum and dividing then?
If i use @var later on (which is also a weighted variance), should i use @ave then?

Thanks, Peter

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