I'm successfully using Welford's method to compute running variance and standard deviation as described many times on Stack Overflow and John D Cook's excellent blog post.
However in the stream of samples, sometimes I encounter a "rollback", or "remove sample" order, meaning that a previous sample is no longer valid and should be removed from the calculation. I know the value of the sample to remove and when it was processed. But I'm using Welford because I can not go back do another pass over all the data.
Is there an algorithm to successfully adjust my running variance to remove or negate a specific previously processed sample?