CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 239
Example 10-1 Variances from the Mean Squared
Follow these calculations where variances from the mean are squared. The variances from the mean were designed to show a consistent
variation from the mean, using both positive and negative values. An equation after this example shows how both SDs and CVs are determined.
Variance from
Mean
Squared
Values
Variance from Mean
Squared
Values
Variance from Mean
Squared
Values
Day One 0.25 0.0625 Day Seven 1.75 3.0625 Day Thirteen –1.00 1.0000
Day Two 0.50 0.2500 Day Eight 2.00 4.0000 Day Fourteen –1.25 1.5625
Day Three 0.75 0.5625 Day Nine 2.25 5.0625 Day Fifteen –1.50 2.250
Day Four 1.00 1.0000 Day Ten –0.25 0.0625 Day Sixteen –1.75 3.0625
Day Five 1.25 1.5625 Day Eleven –0.50 0.2500 Day Seventeen –2.00 4.0000
Day Six 1.50 2.250 Day Twelve –0.75 0.5625 Day Eighteen –2.25 5.0625
NOTE: The variances from the mean value are treated the same, whether values are negative or positive. A negative value, when
squared, becomes a positive value.
The sum of the squared differences for this set of samples is 35.625.
How to Calculate the Standard Deviation
Table 10-2 is a simplified set of procedures to
obtaine a 1 SD value. Working through steps 1
through 7 of Example 10-2 is good practice for
finding the standard deviation values needed for
QC records.
Sample Calculation of a Standard Deviation
Using the steps in Table 10-2, calculate the standard
deviation for the values given in Example
10-2.
Step 1 Calculate the mean value.
Determine the mean from values obtained on days
1 through 20.
FIGURE 10-1 Normal frequency distribution curves.
Source: Delmar/Cengage Learning.
92 + 93 + 92 + 90 + 89 + 87 + 88 + 89 + 88 + 92 + 88 + 91 + 90 + 88 + 87 + 93 + 90 + 92 + 92 + 89
Mean 20 of 90.05 (Round to 90)
Mean Value = 90
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