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A data set (or dataset) is a collection of data, usually presented in tabular form. Each column represents a particular variable. Each row corresponds to a given member of the data set in question. It lists values for each of the variables, such as height and weight of an object or values of random numbers. The data set may comprise data for one or more members, corresponding to the number of rows.

Historically, the term originated in the mainframe field, where it had a well-defined meaning, very close to contemporary computer file. This topic is not covered here.

In the simplest case, there is only one variable, and then the data set consists of a single column of values, often represented as a list.

The values may be numbers, such as real numbers or integers, for example representing a person's height in centimeters, but may also be nominal data (i.e., not consisting of numerical values), for example representing a person's ethnicity. For each variable, the values will normally all be of the same kind. However, there may also be "missing values", which need to be indicated in some way.

In statistics data sets usually come from actual observations obtained by sampling a statistical population, and each row corresponds to the observations on one element of that population. Data sets may further be generated by algorithms for the purpose of testing certain kinds of software.

While the term suggests a relationship to set theory it should not be assumed that a given data set is, in fact, a set in the usual mathematically sense. The rows of a data set need not be distinct, and so a data set is technically a multiset.

Several classic data sets have been used extensively in the statistical literature:

• Iris flower data set - multivariate data set introduced by Ronald Fisher (1936).^[1]
• Categorical data analysis - Data sets used in the book, An Introduction to Categorical Data Analysis, by Agresti are provided on-line by StatLib.
• Robust statistics - Data sets used in Robust Regression and Outlier Detection (Rousseeuw and Leroy, 1986). Provided on-line at the University of Cologne.
• Time series - Data used in Chatfield's book, The Analysis of Time Series, are provided on-line by StatLib.
• Extreme values - Data used in the book, An Introduction to the Statistical Modeling of Extreme Values are provided on-line by Stuart Coles, the book's author.
• Bayesian Data Analysis - Data used in the book, Bayesian Data Analysis, are provided on-line by Andrew Gelman, one of the book's authors.
• The Bupa liver data, used in several papers in the machine learning (data mining) literature.

• StatLib--Datasets Archive
• StatLib--JASA Data Archive

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