How is the Box plot calculated and what do the lines represent?

Each box encloses 50% of the data with the median value of the variable displayed as a line. The top and bottom of the box mark the limits of ± 25% of the variable population. The lines extending from the top and bottom of each box mark the minimum and maximum values within the data set that fall within an acceptable range. Any value outside of this range, called an outlier, is displayed as an individual point.

Outliers are used in the calculations for the Box plot. Removing outliers changes the entire plot. You do not have any control over the marker used to represent outliers.

Median – The data value located halfway between the smallest and largest values.

Upper Quartile (UQ) – The data value located halfway between the median and the largest data value.

Lower Quartile (LQ) – The data value located halfway between the median and the smallest data value.

Interquartile Distance (IQD) – The distance between the Upper and Lower Quartiles (UQ – LQ).

Outliers – Points whose value is either: greater than UQ + 1.5 * IQD or less than LQ – 1.5 * IQD.

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