1. ToolsTalk 2
2. Reports and Statistics
3. Working with the Statistics Tab
4. Calculating Statistics

Calculating Statistics

If Calculate is disabled, make sure that:

• the controller is online.

• SPC configuration is in sync with controller.

• the user is authorized to update the controller.

It is not possible to calculate statistics for results from deleted tightening programs or Virtual Stations, unless the results are still stored on the controller.

Statistics Formulas

Data in a subgroup is calculated as:

• Average:

  

• Range = R = max (X_i) - min (X_i), where i=1...n

• Standard deviation for a subgroup:

  

• Total overall of the standard deviation:

  

  Where N = total number of observations.

When specified number of subgroups to use for calculations has been collected, the SPC function starts calculations to determine the statistical stability.
Calculations are performed through the following steps: (m = number of subgroups)

• Calculation of Average of Average values:

  

• Calculation of Average of Range:

  

• Calculation of Average of Standard deviation:

  

• Calculation of the process capabilities Cp and Cpk with respect to Sigma:

  

  

  Where:

  • is the overall arithmetic average of the observed characteristic values.

  • UTL is the Upper Tolerance Limit.

  • LTL is the Lower Tolerance Limit.

  • The sigma estimation is the mean square standard deviation within the subgroups:

    

• Calculation of the process capabilities Cp and Cpk with respect to Range:

  

  

  

  Where for a normal distribution su = sl = 3.

Calculations for control limits if, in the SPC Configuration section, the Auto calculate switch is set to Yes:

• X-bar R (Mean)
  
  

• X-bar R (Range)
  
  

• X-bar S (Mean)
  
  

• X-bar S (Sigma)
  
  

A₂, A₃, D₃, D₄, B₃, and B₄ are constants (for more information, see the "SPC Calculation Constants" paragraph).

SPC Calculations Constants