Can someone please explain the statistics that isoplot uses to identify outliers for rejection when calculating a weighted mean age for a population?

The manual simply notes that checking the box “permits outlier-rejection using a modified 2-sigma criterion”. My experience has been that samples with a tighter age grouping (low MSWD) tend to yield a more aggressive recognition of outliers, whereas those with a spread of ages (higher MSWD) are typically more conservative and do not reject as many apparent outliers. This is obviously a clue as to what’s happening, but I was hoping for a simple explanation for how this is calculated.

Thanks in advance for anyone’s thoughts . . .