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Nonparametric Statistics

Updated on January 6, 2024
Article byKosha Mehta
Edited byAshish Kumar Srivastav
Reviewed byDheeraj Vaidya, CFA, FRM

What Is Nonparametric Statistics?

Nonparametric statistics is a branch of statistics that deals with data that do not follow a specific distribution. The purpose of nonparametric statistics is to provide statistical methods that are robust and applicable in a wide range of situations without the need to assume a specific distribution or estimate parameters.

Nonparametric statistics

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The importance of nonparametric statistics lies in its ability to handle data that is not normally distributed or may have unknown or complex distributions. It is also useful when the sample size is small, or the data is ordinal or nominal. In addition, nonparametric statistics can be used to test hypotheses, estimate parameters, and make predictions.

Key Takeaways

  • Nonparametric statistics do not require assumptions about the data distribution or population parameters, making them more flexible than parametric methods.
  • Nonparametric methods can be used with nominal, ordinal, interval, or ratio data. These are particularly useful when the data violates the assumptions of parametric methods, such as normality or homogeneity of variance.
  • Nonparametric methods are less sensitive to outliers and can produce accurate results with small sample sizes.
  • Nonparametric methods are generally less powerful than parametric methods when assumptions are met, but they are still useful in situations where parametric methods cannot be used.

Nonparametric Statistics Explained

Nonparametric statistics is a set of methods that can be used to analyze data without making assumptions about the underlying population. This approach is useful when the data does not follow a specific distribution or when the sample size is small. Unlike parametric statistics, nonparametric methods do not require estimating population parameters, such as mean and variance. Instead, they rely on rank-based tests, such as the Wilcoxon rank-sum test, to compare groups or test hypotheses.

Its methods are often used in social sciences, psychology, medicine, biology, and engineering, among other fields. It is a flexible approach that does not make assumptions about the underlying population and uses methods that do not depend on parameters.

Nonparametric methods can also be used to analyze nominal data or ordinal data, meaning the values cannot be measured on a numerical scale. For example, nonparametric methods can be used to compare the proportion of people who prefer a certain brand of cola or to test whether a certain drug effectively reduces pain.

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Assumptions

Assumptions in statistics are conditions or requirements that must be met for a statistical method to be valid. Nonparametric methods do not require strong assumptions about the data distribution or population parameters. However, they still have some assumptions that need to be met, such as random sampling, independence, and symmetry of the distribution. Nonparametric methods also assume that the data is at least ordinal, meaning the values can be ranked or ordered.

Examples

Let us have a look at the example to understand the concept better.

Example #1

Suppose an investor wants to compare Portfolio A’s and Portfolio B’s returns. The investor collects data on each portfolio’s daily returns over one year and tests for normality using a Shapiro-Wilk test. The test reveals that the data is not normally distributed, violating the assumption of parametric methods.

In this case, the investor might use a nonparametric method such as the Wilcoxon rank-sum test or the Mann-Whitney U test to compare the median returns of the two portfolios. These nonparametric methods do not require the data to follow a specific distribution. In addition, they are less sensitive to outliers, making them an appropriate choice for analyzing financial data that may have non-normal or non-symmetric distributions.

Example #2

Suppose a researcher wants to test whether there is a significant difference in the median salaries of employees in two different departments of a company. They collect salary data from a random sample of employees in each department and find that it is not normally distributed. In this case, the researcher can use nonparametric methods, such as the Wilcoxon rank-sum test.

It helps to compare the median salaries of the two departments without assuming any specific distribution or estimating population parameters. The test will give a p-value that indicates whether the difference in median salaries is statistically significant or due to chance. This approach is useful when the data does not meet the assumptions of parametric tests, such as the t-test or ANOVA.

Advantages And Disadvantages

Let us have a look at the pros and cons of the concept:

Advantages of nonparametric statistics include:

  1. Flexibility: Nonparametric methods can be used with data that does not meet the assumptions of parametric methods, such as non-normal or non-homogeneous data.
  2. Robustness: Nonparametric methods are less sensitive to outliers and can produce accurate results even with a small sample size.
  3. Ease of interpretation: Nonparametric methods often use simple statistics, such as ranks or medians, which are easy to understand and interpret.

Disadvantages of nonparametric statistics include:

  1. Less power: Nonparametric methods are generally less powerful than parametric methods when the assumptions are met. Nonparametric tests may require a larger sample size to detect a true difference.
  2. Limited applicability: Nonparametric methods are unsuitable for all data types, such as continuous data that follows a normal distribution.
  3. Limited precision: Nonparametric methods may produce less precise estimates than parametric methods, especially when the sample size is small.

Nonparametric Statistics vs Parametric Statistics

Here are the key differences between the two:

Nonparametric StatisticsParametric Statistics
Do not assume a specific distribution or estimate population parametersAssume a specific distribution and estimate population parameters
Suitable for data that does not meet the assumptions of parametric methods, such as non-normal or non-homogeneous dataSuitable for data that follow a specific distribution, such as normal or binomial
Use rank-based tests or other nonparametric methodsUse mean-based tests, such as t-tests or ANOVA
Are generally less powerful than parametric methods when assumptions are metAre generally more powerful than nonparametric methods when assumptions are met
Are less sensitive to outliers and can produce accurate results with a small sample sizeAre more sensitive to outliers and may require a larger sample size to detect a true difference
Produce less precise estimates than parametric methods, especially with small sample sizes.Produce more precise estimates than nonparametric methods, especially with large sample sizes.
Do not require estimating population parameters, such as mean and variance.Require the estimation of population parameters
It can be used with nominal or ordinal dataRequire interval or ratio data

Frequently Asked Questions (FAQs)

1. Why do we need nonparametric statistics?

Nonparametric statistics are needed when the data does not meet the assumptions of parametric methods, such as normality or homogeneity of variance, or when the data is nominal or ordinal.

2. Is Bayesian parametric or nonparametric statistics?

Bayesian statistics can be considered a parametric approach since it specifies a prior distribution for the population parameter of interest, updated with data to obtain a posterior distribution.

3. Is nonparametric statistics appropriate for n 30?

Nonparametric statistics can be appropriate for any sample size, but they may be particularly useful when the sample size is small or when the assumptions of parametric methods are violated.

4. When to use a nonparametric statistic?

Nonparametric statistics should be used when the data does not meet the assumptions of parametric methods, such as normality or homogeneity of variance, or when the research question involves nominal or ordinal data.

This article has been a guide to what is Nonparametric Statistics. We compare it with parametric statistics, explain its examples, advantages, & assumptions. You may also find some useful articles here –

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