In

statisticsStatistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

, the term

**non-parametric statistics** has at least two different meanings:

- The first meaning of
*non-parametric* covers techniques that do not rely on data belonging to any particular distribution. These include, among others:
*distribution free* methods, which do not rely on assumptions that the data are drawn from a given probability distributionIn probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....

. As such it is the opposite of parametric statisticsParametric statistics is a branch of statistics that assumes that the data has come from a type of probability distribution and makes inferences about the parameters of the distribution. Most well-known elementary statistical methods are parametric....

. It includes non-parametric statistical modelA statistical model is a formalization of relationships between variables in the form of mathematical equations. A statistical model describes how one or more random variables are related to one or more random variables. The model is statistical as the variables are not deterministically but...

s, inferenceIn statistics, statistical inference is the process of drawing conclusions from data that are subject to random variation, for example, observational errors or sampling variation...

and statistical testsA statistical hypothesis test is a method of making decisions using data, whether from a controlled experiment or an observational study . In statistics, a result is called statistically significant if it is unlikely to have occurred by chance alone, according to a pre-determined threshold...

.
*non-parametric statistics* (in the sense of a statisticA statistic is a single measure of some attribute of a sample . It is calculated by applying a function to the values of the items comprising the sample which are known together as a set of data.More formally, statistical theory defines a statistic as a function of a sample where the function...

over data, which is defined to be a function on a sample that has no dependency on a parameterParameter from Ancient Greek παρά also “para” meaning “beside, subsidiary” and μέτρον also “metron” meaning “measure”, can be interpreted in mathematics, logic, linguistics, environmental science and other disciplines....

), whose interpretation does not depend on the population fitting any parametrized distributions. Statistics based on the ranks of observations are one example of such statistics and these play a central role in many non-parametric approaches.

- The second meaning of
*non-parametric* covers techniques that do not assume that the *structure* of a model is fixed. Typically, the model grows in size to accommodate the complexity of the data. In these techniques, individual variables *are* typically assumed to belong to parametric distributions, and assumptions about the types of connections among variables are also made. These techniques include, among others:
*non-parametric regression*Nonparametric regression is a form of regression analysis in which the predictor does not take a predetermined form but is constructed according to information derived from the data...

, which refers to modeling where the structure of the relationship between variables is treated non-parametrically, but where nevertheless there may be parametric assumptions about the distribution of model residuals.
*non-parametric hierarchical Bayesian models*, such as models based on the Dirichlet processIn probability theory, a Dirichlet process is a stochastic process that can be thought of as a probability distribution whose domain is itself a random distribution...

, which allow the number of latent variables to grow as necessary to fit the data, but where individual variables still follow parametric distributions and even the process controlling the rate of growth of latent variables follows a parametric distribution.

## Applications and purpose

Non-parametric methods are widely used for studying populations that take on a ranked order (such as movie reviews receiving one to four stars). The use of non-parametric methods may be necessary when data have a

rankingA ranking is a relationship between a set of items such that, for any two items, the first is either 'ranked higher than', 'ranked lower than' or 'ranked equal to' the second....

but no clear numerical interpretation, such as when assessing preferences. In terms of

levels of measurementThe "levels of measurement", or scales of measure are expressions that typically refer to the theory of scale types developed by the psychologist Stanley Smith Stevens. Stevens proposed his theory in a 1946 Science article titled "On the theory of scales of measurement"...

, non-parametric methods result in "ordinal" data.

As non-parametric methods make fewer assumptions, their applicability is much wider than the corresponding parametric methods. In particular, they may be applied in situations where less is known about the application in question. Also, due to the reliance on fewer assumptions, non-parametric methods are more robust.

Another justification for the use of non-parametric methods is simplicity. In certain cases, even when the use of parametric methods is justified, non-parametric methods may be easier to use. Due both to this simplicity and to their greater robustness, non-parametric methods are seen by some statisticians as leaving less room for improper use and misunderstanding.

The wider applicability and increased robustness of non-parametric tests comes at a cost: in cases where a parametric test would be appropriate, non-parametric tests have less

powerThe power of a statistical test is the probability that the test will reject the null hypothesis when the null hypothesis is actually false . The power is in general a function of the possible distributions, often determined by a parameter, under the alternative hypothesis...

. In other words, a larger sample size can be required to draw conclusions with the same degree of confidence.

## Non-parametric models

*Non-parametric models* differ from

parametricParametric statistics is a branch of statistics that assumes that the data has come from a type of probability distribution and makes inferences about the parameters of the distribution. Most well-known elementary statistical methods are parametric....

models in that the model structure is not specified

*a priori* but is instead determined from data. The term

*non-parametric* is not meant to imply that such models completely lack parameters but that the number and nature of the parameters are flexible and not fixed in advance.

- A histogram
In statistics, a histogram is a graphical representation showing a visual impression of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson...

is a simple nonparametric estimate of a probability distribution
- Kernel density estimation
In statistics, kernel density estimation is a non-parametric way of estimating the probability density function of a random variable. Kernel density estimation is a fundamental data smoothing problem where inferences about the population are made, based on a finite data sample...

provides better estimates of the density than histograms.
- Nonparametric regression
Nonparametric regression is a form of regression analysis in which the predictor does not take a predetermined form but is constructed according to information derived from the data...

and semiparametric regressionIn statistics, semiparametric regression includes regression models that combine parametric and nonparametric models. They are often used in situations where the fully nonparametric model may not perform well or when the researcher wants to use a parametric model but the functional form with...

methods have been developed based on kernelA kernel is a weighting function used in non-parametric estimation techniques. Kernels are used in kernel density estimation to estimate random variables' density functions, or in kernel regression to estimate the conditional expectation of a random variable. Kernels are also used in time-series,...

s, splinesIn mathematics, a spline is a sufficiently smooth piecewise-polynomial function. In interpolating problems, spline interpolation is often preferred to polynomial interpolation because it yields similar results, even when using low-degree polynomials, while avoiding Runge's phenomenon for higher...

, and waveletA wavelet is a wave-like oscillation with an amplitude that starts out at zero, increases, and then decreases back to zero. It can typically be visualized as a "brief oscillation" like one might see recorded by a seismograph or heart monitor. Generally, wavelets are purposefully crafted to have...

s.
- Data envelopment analysis
Data envelopment analysis is a nonparametric method in operations research and economics for the estimation of production frontiers. It is used to empirically measure productive efficiency of decision making units...

provides efficiency coefficients similar to those obtained by multivariate analysisMultivariate analysis is based on the statistical principle of multivariate statistics, which involves observation and analysis of more than one statistical variable at a time...

without any distributional assumption.

## Methods

**Non-parametric** (or

**distribution-free**)

**inferential statistical methods** are mathematical procedures for statistical hypothesis testing which, unlike

parametric statisticsParametric statistics is a branch of statistics that assumes that the data has come from a type of probability distribution and makes inferences about the parameters of the distribution. Most well-known elementary statistical methods are parametric....

, make no assumptions about the

probability distributionIn probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....

s of the variables being assessed. The most frequently used tests include

## General references

- Corder, G.W. & Foreman, D.I. (2009)
*Nonparametric Statistics for Non-Statisticians: A Step-by-Step Approach*, Wiley ISBN 9780470454619
- Gibbons, Jean Dickinson and Chakraborti, Subhabrata (2003)
*Nonparametric Statistical Inference*, 4th Ed. CRC ISBN 0824740521
- Wasserman, Larry (2007)
*All of nonparametric statistics*, Springer. ISBN 0387251456
- Bagdonavicius, V., Kruopis, J., Nikulin, M.S. (2011). "Non-parametric tests for complete data", ISTE&WILEY: London&Hoboken. ISBN 9781848212695