python - Computing Standard Deviation in a stream

Question

Using Python, assume I'm running through a known quantity of items I, and have the ability to time how long it takes to process each one t, as well as a running total of time spent processing T and the number of items processed so far c. I'm currently calculating the average on the fly A = T / c but this can be skewed by say a single item taking an extraordinarily long time to process (a few seconds compared to a few milliseconds).

I would like to show a running Standard Deviation. How can I do this without keeping a record of each t?

Answer 1

As outlined in the Wikipedia article on the standard deviation, it is enough to keep track of the following three sums:

s0 = sum(1 for x in samples)
s1 = sum(x for x in samples)
s2 = sum(x*x for x in samples)

These sums are easily updated as new values arrive. The standard deviation can be calculated as

std_dev = math.sqrt((s0 * s2 - s1 * s1)/(s0 * (s0 - 1)))

Note that this way of computing the standard deviation can be numerically ill-conditioned if your samples are floating point numbers and the standard deviation is small compared to the mean of the samples. If you expect samples of this type, you should resort to Welford's method (see the accepted answer).