Generally, control charts will define upper and lower limits based on the standard deviation of the process.  The "standard deviation" is a measure of how much we might expect a score to be above or below the shooter's average.  The farther above average (or below) a competitor shoots the more likely it is that something interesting is going on; something we might want to understand.  These boundaries are called the Upper Control Limit (UCL) and the Lower Control Limit (LCL).  

However, when averages get high enough, they start to run up against a concrete upper limit (100%).  In this situation, the traditional approach to setting limits no longer makes sense.  Clearly, it is far easier to bring an average up from 85% to 90% than it is to bring an average up from 95% to 100%.  In situations like these, we select limits that make sense in the context of our study.

For trap shooters, in the example above, we set the LCL to be at the top of the B class and the UCL to be top of the A Class.

Statistical Process Control (SPC) is part of a larger body of inquiry concerned with quality control.  SPC's value to Clay Metrics comes from three areas:

  1. Use of Statistics to draw conclusions
  2. Use of charts (e.g., "control charts") to report results
  3. Focus on continuous improvement in the tradition of Kaizen
  4. Design of experiments.

SPC has data collection as it's life blood.  

The Scientific Study of Clay Target Shooting