|Mean and Standard Deviation|
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 a lot of control material. New control values must be calculated and acceptable ranges established for each new lot of control materials. Ideally, at least 20 samples should be tested over time for good statistical data.The mean is calculated by adding all of the 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: That is, calculate the deviation from the mean for each point, square 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 -1, 1, -3, 3, and 0. The squared deviations are 1, 1, 9, 9, and 0. The standard deviation is therefore:
|Random and Systematic Error|
There are two types of error that one should look for when examining quality control charts. Random errors are statistical fluctuations (in either direction) in the measured data due to the precision limitations of the measurement device.Systematic errors, by contrast, are reproducible inaccuracies that are consistently in the same direction. Systematic error is more subtle and harder to detect.