Quality Control of Laboratory Measurements

Multi-Rule Shewhart Procedure Applied to Levey-Jennings Control Chart

  • The multi-rules procedure published by Westgard et. al. (1981)*, is used for Levey-Jennings control chart to detect trends or shifts by examining individual values (to determine the status of the measuring system).
  • These rules are based on σ and are hence calculated without regard to constant sample sizes.
  • * Westgard, J.O., Barry, P.L., and Hunt, M.R. (1981). A Multi-Rule Shewhart Chart for Quality Control in Clinical Chemistry.

Multi-Rule Shewhart Procedure for Detecting Trends of Shifts

  • Rule 12s represents the control rule when one point is outside 2σ.

  • Rule 13s warning will be triggered when one point is outside 3σ.

  • Rule 22s warning rule will be triggered when two consecutive points are outside 2σ on the same side of the mean line.

  • Rule 44s will be triggered when the range of two points is larger than 4σ.

  • Rule 41s will be triggered when four consecutive points exceed 1σ on the same side of the mean line.

  • Rule 10X(bar) will be applied when ten consecutive points are all above or below the mean line.