The chance of finding one or more "outlier" by Tukey's rule in data sampled from a Gaussian distribution depends on sample size. ▪ The values that are plotted individually are sometimes called outliers, but "outlier" is defined differently by Grubbs test or some other outlier test. ▪ When the Tukey method is used to create the whiskers, the ends of the whiskers are sometimes called the inner fences. ▪ Why 1.5IQR? There is no statistical rationale it is simply how Tukey decided to do it, and he invented the idea of box-and-whisker plots. Otherwise stop the lower whisker at the lowest value greater than the 25th percentile minues 1.5IQR, and plot any values that are greater than this as individual points. If this value is less than the smallest value in the data set, draw the lower whisker to the smallest value. Calculate the 25th percentile minus 1.5IQR. Otherwise stop the upper whisker at the the largest value less than the sum of the 75th percentile plus 1.5IQR, and plot any values that are greater than this as individual points.ģ. If this value is greater than (or equal to) the largest value in the data set, draw the upper whisker to the largest value. Add the 75th percentile plus 1.5 times IQR. Calculate the inter-quartile distance (the difference between the 25th and 75th percentiles). How the Tukey method plots whiskers and outliersġ. ▪ Choose a fill pattern for the box, and choose the design (pattern) and color. ▪ Choose the border color and thickness, and fill the box with color. This method plots whiskers down to the minimum and up to the maximum value, but also plots each individual value as a point superimposed on the graph.
Points below and above the whiskers are drawn as individual points. The whiskers are drawn down to the 10th percentile and up to the 90th. The whiskers go down to the smallest value and up to the largest. Prism offers six ways to create whiskers in box-and-whiskers plots:
You can not choose a different value, but Prism also lets you put a "+" at the mean. The line in the middle of the box is plotted at the median. Prism uses a standard method, but a different method than Excel uses. Surprisingly, there are multiple ways to compute these percentile values. These limits are sometimes called the hinges of the plot. The box always extends from the 25th to 75th percentiles. Prism lets you create box-and-whisker plots from stacks of values entered into a Column table, or side-by-side replicates entered into an XY or Grouped table. Before creating a box-whiskers plot, consider a violin plot instead.
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