What does non-parametric statistics refer to?

Non-parametric statistics refers to statistical methods that do not rely on any assumptions about the parameters of the population distribution from which the samples are drawn. This means that these tests are often referred to as "distribution-free" because they can be used regardless of the underlying distribution of the data. For example, non-parametric tests are useful when the data are ordinal, have outliers, or are not normally distributed.

The focus of non-parametric methods is on the ranks or order of the data rather than their raw values, which allows for flexibility in their application across various types of data. Analysts often choose non-parametric tests when sample sizes are small or when the data fails to meet the assumptions necessary for parametric tests (such as normality or homogeneity of variance). Thus, the correct description of non-parametric statistics accurately captures its foundation in avoiding reliance on population value assumptions, distinguishing it from parametric methods.