Regarding pd.DataFrame.std() function
It looks like it calculates an adjusted value of std, i.e. a squared deviation from the mean, (x_i - x_bar) *\* 2, divided by n-1 instead of n.
It is a reasonable measure for cases when n <= 50 allowing to avoid underestimation of std. But in cases when n > 50 actually there is no difference what number is used in the denominator, n or n-1. I guess you need to take this into account.

Many thanks for your excellent piece of work.

B.R., Andre Logunov, Russia
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