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.
Example:
Given nby2 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.
Reference:
http://www.bmj.com/content/309/6960/996.full
