CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
Table 10-2 Calculating Standard Deviations
Step Activity Example of How to Obtain Values
1. Determine the mean of the control
values.
2. Calculate deviations (±) from the
mean.
When 10 laboratory values are obtained, add them all
together. Then divide the sum of the ten values by 10.
This is the mean.
Calculate the amount each laboratory value deviates
from the mean, plus or minus.
3. Square the deviations from the mean. Multiply each variance by itself.
4. Add all the squared deviations. Calculate the sum of all the squared deviations.
5. Divide sum of squared differences
by n – 1.
6. Take the square root of the
quotient from step 5.
The sum of the squared differences is divided by the
total number of values (procedures) minus 1.
Calculate the square root of the fi gure obtained
Example 10-2 Graph for Normal Control in Daily Glucose Tests
A normal level control (80–105 mg/dL) was tested at the beginning of the morning shift at a local
hospital. The reagent used was of the same lot number, the instrument operator was the same, and
all conditions were controlled to ensure the results would not be affected by environment or by any
condition that could be controlled. These simulated daily results are taken from the Levey-Jennings chart
in Example 10-3.
Day 1 92 Day 6 87 Day 11 88 Day 16 93
Day 2 93 Day 7 88 Day 12 91 Day 17 90
Day 3 92 Day 8 89 Day 13 90 Day 18 92
Day 4 90 Day 9 88 Day 14 88 Day 19 92
Day 5 89 Day 10 92 Day 15 87 Day 20 89
Step 2 Calculate deviations from the mean (negative and positive numbers are
treated the same) by subtracting each of the 20 values from the mean.
+2, +3, +2, 0, –1, –3, –2, –1, –2, +2, –2, +1, 0, –2, –3, +3, 0, +2, +2, –1
Step 3 Square the deviations from the mean.
4.0, 9.0, 4.0, 0, 1.0, 9.0, 4.0, 1.0, 4.0, 4.0, 4.0, 1.0, 0, 4.0, 9.0, 9.0, 0, 4.0, 4.0, 1.0
Step 4 Add all the squared deviations.
76
Step 5 Divide the sum of squared differences by n – 1.
76 divided by (20 – 1) = 4.00
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