Determining the Type of Error

Determining the type of error, random or systematic, is a good first step when investigating QC results. Different rules are sensitive to different types of errors. For example:

• The 1_3s and the R_4s rules usually indicate increased random error because they test the tails or width of a distribution
• The 2_2s, 4_1s, and 10_x rules usually indicate systematic error because they examine consecutive QC results that exceed the same limit.

Levey-Jennings Charts also indicate the type of error.

Tip: When possible, always identify the type of error before trying to identify the cause of the problem. Further classification of systematic error as a shift or trend is also useful.