The non-parametric difference score is introduced. It is a workable solution to the problem of analyzing change over two waves (i.e., a pretest-posttest design) when the measures themselves vary over time. An example highlighting the solution’s implementation is provided, as is a discussion of the solution’s assumptions, strengths, and limitations.
Lloyd, Jennifer E. V. and Zumbo, Bruno D.
"The Non-Parametric Difference Score: A Workable Solution for Analyzing Two-Wave Change When The Measures Themselves Change Across Waves,"
Journal of Modern Applied Statistical Methods:
2, Article 7.
Available at: http://digitalcommons.wayne.edu/jmasm/vol6/iss2/7