Establish Calibration Intervals for Parameters 
Based on deviations between as-left and as-found values and the time elapsed between successive tests or calibrations, intervals can be determined for monitoring parameters that are consistent with  desired levels of confidence that parameters are in-tolerance.  The SPC Interval Worksheet can be used to compute a recommended test or calibration interval commensurate  with a desired confidence level or Reliability Target.  This worksheet is activated by clicking Compute Interval on the menu of the Process Control Chart screen.

The projections on the SPC Interval Worksheet differ from those on the Process Control Chart screen in that the upper and lower projection limit intersections with the upper control limit (UCL) and lower control limit (LCL) can be referenced to a particular confidence level, whereas, on the Control Chart screen, the projection limits are ± one sigma.
 
The SPC Interval Worksheet also shows the Drift Rate for the process and allows setting the time units.  In addition, when SPCView is used to estimate intervals for parameters, the Initial Uncertainty and Initial Value can be set to zero or some other value appropriate for the start of the parameter's interval.  This allows projecting an interval from a beginning point where a parameter adjustment or some other renewal has taken place.  Initial values may also be corrected as needed.
 
Computing Recommended Intervals
Entering a percent confidence in the EOP Reliability box (End of Period) and pressing the Enter key causes an interval and an accompanying projected value to be computed.  The interval corresponds to the elapsed time at which the confidence that the parameter value or deviation will be within the indicated tolerance limits is equal to the EOP Reliability.
 
This interval is not necessarily the same as the projected intercept.  The latter is the time required for a projected parameter value or deviation to cross a tolerance limit. The recommended interval is the time required for the uncertainty in this value or deviation to grow to the point where our confidence in its being in-tolerance has dropped to a level equal to the EOP Reliability.

Interval Analysis Reports
A full report of the analysis of a parameter interval may be obtained by selecting Print from the menu bar menu.  The Interval Analysis Report includes the technical information shown on the worksheet along with the plotted interval projection.