v = nanstd(X [, opt [, dim]]);
nanstd is identical to the std function except that NaN values are
ignored.  If all values are NaN, the std is returned as NaN. If there
is only a single non-NaN value, the std is returned as 0.

0:  normalizes with N-1 [default]
provides the square root of best unbiased estimator of the variance
1:  normalizes with N,
this provides the square root of the second moment around the mean

See also: nanmin, nanmax, nansum, nanmedian, nanmean