this is most probably due to the fact that 2 = sqrt(5 - 1), a correction to
variance reduction incurred by the overlapping nature of the folds. the
bootstrap book contains more info on how to calculate these for different
cases of splitting.

hth,
michael

On Thursday, February 5, 2015, Sebastian Raschka <se.rasc...@gmail.com>
wrote:

> Hi,
>
> I am wondering why the standard deviation of the accuracy estimate is
> multiplied by 2 in the example on
> http://scikit-learn.org/stable/modules/cross_validation.html; it would be
> nice if someone could explain it to me.
>
> The relevant excerpt from the page linked above:
>
> >>> clf = svm.SVC(kernel='linear', C=1)
> >>> scores = cross_validation.cross_val_score(
> ... clf, iris.data, iris.target, cv=5)
> ...
> >>> scores
> array([ 0.96..., 1. ..., 0.96..., 0.96..., 1. ])
> The mean score and the standard deviation of the score estimate are hence
> given by:
> >>>
> >>> print("Accuracy: %0.2f (+/- %0.2f)" % (scores.mean(), scores.std() *
> 2))
> Accuracy: 0.98 (+/- 0.03)
>
>
> Best,
> Sebastian
>
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