# Statistics - (Non) Parametrics (method|statistics)

Parametric statistics is a branch of statistics.

A Parametric method, such as t-tests is a method that is based on fixed assumption that the sample data comes from a population that follows a probability distribution (normal, poisson, ) based on a fixed set of parameters.

Ie: the assumed distribution is fix. For instance, for the normal distribution, the parameters $\mu$ and $\sigma$ are fixed in its density

Conversely a non-parametric model differs precisely in that the parameter set (or feature set in machine learning) is not fixed and can increase, or even decrease if new relevant information is collected.