With certain sequences of input values the Variance PtByPt.vi can generate negative values. This is due to the finite precision in the calculations.
Because it is very common to calculate the standard deviation by taking the square root of the variance, this behavior can take you by surprise as the result will be NaN's and not almost 0 as you would expect. Depending on the data, this can be frequent or extremely in frequent. In general - a nuisance to debug.
The cure is simple: Take the Abs() value before the square root. Attached is a vi which demonstrates the case.
I would suggest to include this correction (or similar) into the "Variance PtByPt.vi" because it does give rise to unexpected and hard to find errors.
By definition - variance cannot be negative.
