# Probabilistic Variables: What is the difference between control and noise variables?

A probabilistic variable is completely described using a statistical distribution. The statistical distribution defines the mean or nominal value as well as the variation around this nominal value.

A distinction is made between control and noise variables:

**Control variables:**Variables that can be controlled in the design, analysis, and production level; for example: a shell thickness. It can therefore be assigned a nominal value and will have a variation around this nominal value. The nominal value can be adjusted during the design phase in order to have a more suitable design. A discrete variable is a special case of a control variable.**Noise variables:**Variables that are difficult or impossible to control at the design and production level, but can be controlled at the analysis level; for example: loads and material variation. A noise variable will have the nominal value as specified by the distribution, that is follow the distribution exactly.