expain what is westgard rule 12s One control observation exceeding the mean +2s. A warning rule that initiates testing of control data by other rules. Patient samples can still be run - only rule where this is possible 13s One control observation exceeding the mean + 3s. Allows high sensitivity to random error (incorrect pipetting etc) 22s Two control observations consecutively exceeding the same +2s or -2s. Allows high sensitivity to systemic error R4s two measurements in the same run have a standard deviation difference 41s Four consecutive control observations exceeding +1s or -1s. This allows the detection of systemic error 10x 10 measurements in a row are on the same side of the mean