Many natural phenomena and man-made processes are observed to have normal distributions, or can be closely represented as normally distributed.

For example, the length of a machined part is observed to vary about its mean due to:

temperature drift, humidity change, vibrations, cutting angle variations, cutting tool wear, bearing wear, rotational speed variations, fixturing variations, raw material changes and contamination level changes

If these sources of variation are small, independent and equally likely to be positive or negative, the length will closely approximate a normal distribution.