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Anscombe devised four data sets to illustrate the importance of graphical examination of the data in partnership with numerical statistical summaries. Each of the data sets consists of 11 values of an independent variable x and a dependent variable y. Three of the data sets contain the same values of x.
Source: F.J. Anscombe (1973). Graphs in Statistical Analysis. The American Statistician 27(1), 17-21.