Michael;

Yes, I understand it.

Simple probability is based on time. If your window takes up 20% of the time it has a 20% chance of being right. or it should catch 20% of the quakes. It's a pretty crude estimate and clusters mess it up.

The Jones probability is based on seismic history and tells you whether a window of a given size should catch at least one quake. Clusters have little impact because you count hit windows, not quakes.

Roger