The 'total' is the sum of the values in the column. The 'good' is simply the number of rows in the virtual table. Minimum and maximum values, the total signal, and the number of good values are also reported. In the image context, the 'total' is the sum of the pixel values in the image, and the 'good' is the number of pixels. An iterative, sigma-clipping, algorithm can be used to reduce the effect of outliers on the computed statistics.
Options exist to choose which statistics to compute if the execution time is a factor.
The sigma-clipping algorithm calculates the mean and standard deviation of the data, then removes all points which are more than nsigma times the standard deviation away from the mean. If a column in a table contains a WCS transform, then the clipping is also performed on the transformed data. In particular, if intable.fits is a photon event file, the 'good' value will give the number of counts satisfying the filter. Note that the averages are actually the weighted centroids and the sigmas are the weighed standard deviations. This is repeated until either no more values are rejected, or 10 iterations have been performed.