CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 241
Step 6 Take the square root of the quotient determined by dividing the sum of
squared differences by n – 1.
Square root of 4.00 = 2.00 (1 SD)
Step 7 Calculate 2 SDs and 3 SDs from 1 SD.
1 SD = 2.00 2 SDs = 4.00 3 SDs = 6.00
Equation for Calculating a Standard Deviation
and a Coefficient of Variation
The following formula is a simplified explanation of the calculations just
explained at length, using the values provided in Example 10-2. Both the standard
deviation and the coefficient of variation are indicated. Consult the lengthy
explanation and then recalculate the values by using the simple equation presented
here.
Calculating the Standard Deviation (SD)
The variance (SD 2 ) of the samples in Example 10-2 is:
SD 2 = 76.0/20 (total values) – 1 or (n – 1) = 76/19 = 4.00
The square root of 4.00 = 2.00, or 1 SD
2 SDs = 2 × 2.00, or 4.00
How to Calculate the Coefficient of Variation
The coefficient of variation (CV) is always expressed in a percentage form. It is
based on the percent value that each individual control value differs from the
mean. It is another way to express the degree of deviation from the mean.
Coefficient of Variation
Coefficient of variation (CV) = SDs/sample mean × 100%
For the set of sample values from Example 10-2,
CV = SD (2.00) divided by the mean (90) × 100 = 2.22%
CUSUM: Another Quality Control Tool
A term that is not often heard in the forefront of quality control is that of a
cumulative sum (CUSUM) control chart in which the cumulative sums of deviations
from the mean value are recorded manually or by computer programs within
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