1. 4
  1.  

  2. 1

    The main description of combinatorial testing and its benefits are here:

    http://csrc.nist.gov/groups/SNS/acts/index.html

    The main paper on this work is paywalled per their wiki:

    https://github.com/dakusui/jcunit/wiki

    The submitted link has a graphical view of what they do further down on the page. Plenty of description, too.