[Numpy-discussion] please change mean to use dtype=float

Christopher Barker Chris.Barker at noaa.gov
Wed Sep 20 11:56:53 CDT 2006

Sebastian Haase wrote:
> The best I can hope for is a "sound" default for most (practical) cases...
> I still think that 80bit vs. 128bit vs 96bit is rather academic for most 
> people ...  most people seem to only use float64 and then there are some 
> that use float32 (like us) ...

I fully agree with Sebastian here. As Travis pointed out, "all we are 
talking about is the default". The default should follow the principle 
of least surprise for the less-knowledgeable users. Personally, I try to 
always use doubles, unless I have a real reason not to. The recent 
change of default types for zeros et al. will help.

clearly, there is a problem to say the default accumulator for *all* 
types is double, as you wouldn't want to downcast float128s, even if 
they are obscure. However, is it that hard to say that the default 
accumulator will have *at least* the precision of double?

Robert Kern wrote:
> Let me offer a third path: the algorithms used for .mean() and .var() are 
> substandard. There are much better incremental algorithms that entirely avoid 
> the need to accumulate such large (and therefore precision-losing) intermediate 
> values.

This, of course, is an even better solution, unless there are 
substantial performance impacts.


Christopher Barker, Ph.D.
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Chris.Barker at noaa.gov

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