# Need calculate and update iteratively Standard deviation

I have a list of Double Values(List of distances form a fixed central point), and I repeatedly chuck out some values(based on simple rule of largest value) and see the change in SD.

I would like to calculate it using a rapid/iterative way to calculate SD after removal of a point.

Have read about different incremental approaches to calculate SD/Variance, but all seem to be for updating SD/Variance after addition of a new data value.

http://en.wikipedia.org/wiki/Standard_deviation#Rapid_calculation_methods

https://stackoverflow.com/questions/895929/how-do-i-determine-the-standard-deviation-stddev-of-a-set-of-values

https://stackoverflow.com/questions/1174984/how-to-efficiently-calculate-a-running-standard-deviation

Take the observation you want to remove. Imagine you just added that observation as the $n$-th observation (of $n$), using one of the algorithms you point to. Now undo those steps you just did, to take it back to the way it was just before it was added (the variance as at $n-1$ observations).