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Figure 1. Box plot of data from the Michelson–Morley experiment

In descriptive statistics, a box plot or boxplot is a method for graphically depicting groups of numerical data through their quartiles.

Box plots may also have lines extending from the boxes (whiskers) indicating variability outside the upper and lower quartiles, hence the terms box-and-whisker plot and box-and-whisker diagram.

Outliers may be plotted as individual points.

Box plots are non-parametric: they display variation in samples of a statistical population without making any assumptions of the underlying statistical distribution (though Tukey’s boxplot assumes symmetry for the whiskers and normality for their length).

The spacings between the different parts of the box indicate the degree of dispersion (spread) and skewness in the data, and show outliers.

In addition to the points themselves, they allow one to visually estimate various L-estimators, notably the interquartile range, midhinge, range, mid-range, and trimean.

Box plots can be drawn either horizontally or vertically. Box plots received their name from the box in the middle.

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