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From Sean Owen <>
Subject Re: Questions about PearsonCorrelation on a example
Date Tue, 23 Jun 2009 22:40:57 GMT
It doesn't have to do with the mean preference value being 0.

The problem is that these users overlap in only one item, and so,
Pearson cannot compute a similarity since it cannot define a
correlation coefficient. A correlation would require at least two data
points. Add some more data and you will get non-NaN values.

On Tue, Jun 23, 2009 at 6:38 PM, charlysf<> wrote:
> Hello,
> I use PearsonCorrelation, and I would like to understand why on my example,
> I have a NaN.
> I use PearsonCorrelation for an Item Basis recommender, and I of course
> expect, to have a similarity between the two items, because they are linked
> to a same user.
> But in fact, I have a NaN.
> I saw that the mean of the data should be 0, but or me, 0 is the neutral
> score, and I can't be sure that for each user, their mean is 0, is it
> important ? I don't really understand why.
> Thank you.
> --
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