Mean can be defined as the average of the data points.
Standard deviation (SD) is a measure of imprecision. It indicates the variability or dispersion around the mean.
Together, mean and SD determine acceptable ranges for many control materials. New control values must be calculated, and acceptable ranges must be established for each new lot of control materials. Ideally, at least 20 samples should be tested for good statistical data.
The mean is calculated by adding all values and dividing by the number of values. The formula is
For example, suppose you wanted to find the mean of the values 4, 6, 2, 8, and 5. The mean is
The standard deviation (abbreviated s or SD) is calculated according to the following formula:
Calculate the deviation from the mean for each point, divide those results, sum them, divide by the number of points minus one, and finally take the square root.
For example, the deviations from the mean in the above example are:
4-5= -1
6-5= 1
2-5= -3
8-5= 3
5-5= 0
The squared deviations are 1, 1, 9, 9, and 0. The standard deviation is therefore:
