Many physical and biological processes are well-modeled by a roughly bell-like distribution. This curve is called the Gaussian, bell curve, or normal distribution. The normal distribution has the following characteristics:
- 68.3% of the area lies between the mean (x¯ ) minus one standard deviation (1 SD) and x¯ plus 1 SD;
- 95.5% of the area lies between x¯ - 2 SD and x¯ + 2 SD; and
- 99.7% of the area lies between x¯ - 3 SD and x¯ + 3 SD.
For example, if a particular control gives a mean test result of 5.6 with an SD of 0.8, then 95.5% of future control test values will be in the range of 4.0–7.2, two standard deviations of the mean. Values outside these limits may not be acceptable (depending on the laboratory's QC rules) and could indicate a problem with the measuring system.
