title: Divide set into subsets with equal number of elements
body: <p>For the purpose of conducting a psychological experiment I have to divide a set of pictures (240) described by 4 features (real numbers) into 3 subsets with equal number of elements in each subset (240/3 = 80) in such a way that all subsets are approximately balanced with respect to these features (in terms of mean and standard deviation).</p>
  
  <p>Can anybody suggest an algorithm to automate that? Are there any packages/modules in Python or R that I could use to do that? Where should I start?</p>