Shooting Group Statistics

Question

What does the 'Mean Radius, Max Spread' actually mean?

Shooting Group Statistics calculated by TargetScan can be defined as:

Mean Point of Impact (MPI) - the centre of the group. This is visualised as a small red cross on the target plot.
Windage - Horizontal (left/right) distance of the Mean Point of Impact from the target centre.
Elevation - Vertical (up/down) distance of the Mean Point of Impact from the target centre.
Mean Radius (MR) - The mean distance of a shot hole from the Mean Point of Impact. This is visualised as the red circle on the target plot. This a quite good metric of the grouping. TargetScan actually calculates unbiased estimate of MR from the sample.
Max Spread - The largest distance between any two shots "centre-to-centre", i.e. for a single shot group it is zero. Even though this is not as good metric of the grouping as the Mean Radius TargetScan calculates this because traditionally this was the only metric practical to measure at the range. This is also known as 'Group Size' or 'Extreme Spread', but sometimes it is calculated as a distance "edge-to-edge" (then for a single shot group it is equal to the caliber).