Many physical and biological processes are well-modeled by a distribution having a roughly bell-like shape. 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 x̄ - 1s and x̄ + 1s
• 95.5% of the area lies between x̄ - 2s and x̄ + 2s
• 99.7% of the area lies between x̄ - 3s and x̄ + 3s

For example, if a certain control gives a mean test result of .56 with standard deviation .8, then 95.5% of future tests on that control will be in the range .40 - .72, within 2 standard deviations of the mean. This sets limits on the range of values produced by the instrument on that control that will be considered acceptable and define which values may not acceptable and could indicate a problem.