One can easily split continuous data into two equally sized groups using
g = x > median(x);
this simple function generalises this idea to more than two roughly equal groups using quantiles/percentiles. It returns a group indicator vector of the same length as the input vector suitable for use with functions like boxplot.
Given n-by-2 matrix of data X, show distribution of second variable within first variable's quartiles
g = group_equally(X(:, 1), 4);
boxplot(X(:, 2), g)
The function is similar to the Stata command "xtile", though might differ due to different definitions of percentiles; this function uses the definition from the reference below.
Ged Ridgway (2021). Group Equally (https://www.mathworks.com/matlabcentral/fileexchange/28937-group-equally), MATLAB Central File Exchange. Retrieved .
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