Table 1.
Definitions of Westgard quality control rules
| Control rule | Definition | Type of error detected |
|---|---|---|
| 12s | One control observation exceeds control limits set as the mean ± 2SD. This is the “warning” rule for a Shewhart chart signaling need for additional inspection of control data using additional control rules. | __ |
| 13s | A run is rejected when a single control measurement exceeds control limits set as the mean ± 3SD. This is the usual “action” or rejection limit on a Shewhart control chart. | Random |
| 22s | A run is rejected when two consecutive control measurements exceed the same limit, either the mean + 2SD or the mean–2SD. This rule can be applied “within” materials (consecutive observations on the same control material) or “across” materials (consecutive observations on different control materials). | Systematic |
| R4s | A run is rejected when the range or difference between two control observations within a run exceeds 4SD. In other words, one control observation exceeds a + 2SD limit and the second control observation exceed a–2SD limit for a total of 4SD difference between them. | Random |
| 41s | A run is rejected when four consecutive control observations exceed the same limit, which is the mean + 1SD or the mean–1SD. Consecutive observations can occur within one control material or across control materials. | Systematic |
| 10Mean | A run is rejected when 10 consecutive control observations fall on the same side of the mean. Consecutive observations can occur within one control material or across control materials. | Systematic |