Question: I need to know if a number compared to a set of numbers is outside of 1 stddev from the mean, etc..

Answer: 
You can avoid making two passes over the data by accumulating the mean and mean-square

```
cnt = 0
mean = 0
meansqr = 0
loop over array
    cnt++
    mean += value
    meansqr += value*value
mean /= cnt
meansqr /= cnt

```

and forming

```
sigma = sqrt(meansqr - mean^2)

```

A factor of `cnt/(cnt-1)` is often appropriate as well.

BTW-- The first pass over the data in [Demi](https://stackoverflow.com/questions/895929/how-do-i-determine-the-stddev-of-a-set-of-values/896070#896070) and [McWafflestix](https://stackoverflow.com/questions/895929/how-do-i-determine-the-stddev-of-a-set-of-values/895939#895939) answers are hidden in the calls to `Average`. That kind of thing is certainly trivial on a small list, but if the list exceed the size of the cache, or even the working set, this gets to be a bid deal.
We may be able to use statistics module in Python. It has stedev() and pstdev() commands to calculate standard deviation of sample and population respectively.

details here: <https://www.geeksforgeeks.org/python-statistics-stdev/>

import statistics as st
print(st.ptdev(dataframe['column name']))