Total error and total allowable error are useful when choosing the SPC rule(s) to apply to a test. For example, SPC rules can identify a test exceeding a quality specification (TEa). Total error for a test includes both bias and imprecision at a 99% confidence interval. Use the following formula to calculate the TE:
TEa specifications are available from several sources as described in "Determine Quality Requirements for the Test." After choosing a TEa, calculate the TE budget. Use the following formula to calculate the TE budget:
With the optional Westgard Advisor™ online, choose a TEa and the program suggests SPC rules based on test data and Unity™ Interlaboratory Program information.