QC calculations

Answer three multiple-choice questions about control statistics: calculate a standard deviation, calculate sigma, and identify a multirule violation.

Question 1 of 3

A control's ±2 SD interval is 80 to 110 mg/dL. What is 1 SD?

7.5 mg/dL

Correct. The full interval spans 4 SD, from 2 SD below the mean to 2 SD above it. Dividing 30 mg/dL by 4 gives 7.5 mg/dL.

15 mg/dL

Incorrect. The distance from the mean of 95 mg/dL to either limit is 15 mg/dL, which represents 2 SD. Dividing that distance by 2 gives 7.5 mg/dL.

30 mg/dL

Incorrect. Subtracting 80 from 110 gives the full width of the interval. That width spans 4 SD, so 1 SD is 30 ÷ 4 = 7.5 mg/dL.

Question 2 of 3

A procedure has CV 2%, bias −2%, and a laboratory-adopted allowable total error of 10%. What is its sigma metric at this concentration?

Correct. Use the magnitude of bias: (10 − 2) ÷ 2 = 4.

Incorrect. (10 − (−2)) ÷ 2 incorrectly treats negative bias as extra allowable error. Either bias direction consumes the allowance.

Incorrect. 10 − 2 gives the remaining allowance in percentage points. Divide it by CV 2% to obtain sigma 4.

Question 3 of 3

Two controls in the same run are +2.4 SD and −2.3 SD from their respective means. Which rejection rule is met?

2₂s

Incorrect. 2₂s requires consecutive results beyond the same 2 SD limit. These deviations point in opposite directions.

R₄s

Correct. One result is above +2 SD and the other below −2 SD in the same run. Their separation is 4.7 SD.

1₃s

Incorrect. Neither result exceeds a 3 SD limit. Their within-run separation meets R₄s.