Normal (or Gaussian) distribution

It is common in processes for most measurements to cluster around a central value, with less and less measurements occurring further away from this center. For example, the distribution of holes across the target will gradually spread out from a central, most common placement, as below.

The bell-shaped curve occurs surprisingly often and is consequently called a Normal distribution (or Gaussian distribution, after its discoverer, or simply Bell-curve) and has some very useful properties which can be used to help variation be understood and controlled.

See also: