AimScan

Group Statistics: MPI, Mean Radius and Extreme Spread

The score tells you what happened. Group statistics tell you why.

AimScan session screen with a scored air pistol target, group centre and statistics
The session plot: the red cross is the MPI, the red circle the mean radius.

Mean point of impact (MPI)

The MPI is the average position of all shots in a group: the centre of your group. AimScan shows it as a red cross and splits it into:

  • Windage: how far the group centre is left (L) or right (R) of the target centre.
  • Elevation: how far it is high or low.

The MPI is what you correct with your sights. If your session windage is 3 mm right, move your point of impact 3 mm left. Misses are left out.

Mean radius (MR)

The mean radius is the average distance of each shot to the MPI. It is the best single number for precision, because it uses every shot, not just the two worst ones. Lower is better.

AimScan uses the unbiased estimate, which corrects for small groups:

MR = (Σ distance to MPI ÷ n) × √(n ÷ (n − 1))

Shown as a red circle on the plot. Roughly half of your shots will land inside it.

Extreme spread (ES)

The largest centre-to-centre distance between any two shots in the group. It is the classic "group size" that rifle shooters quote. It is easy to understand, but it depends on only two shots, so it jumps around more than the mean radius.

Per target, series and session

On the session screen, switch between Group (all shots in the session) and Target (one card). Use the session numbers for sight corrections and the per-target numbers to see how you held up during the match.

Units

All distances are shown in millimetres or inches. Change this in Settings, Units.

Using the numbers

  • MPI far from centre, small MR: you shoot consistently, adjust your sights.
  • MPI near centre, large MR: your sights are fine, work on position, trigger and follow-through.
  • MPI moves between sessions: check the windage and elevation charts before you touch the sights. One session is not a trend.