Alfie Abdul-Rahman, M Chen, and David H. Laidlaw.
 <b>A survey of variables used in empirical studies for
  visualization</b>.
 In <em>Foundations of Data Visualization</em>. LNCS-Springer, 2019.
  <a href="http://vis.cs.brown.edu/docs/pdf/g/Abdul-2019-SVU.pdf.html">(pdf)</a>
  <a href="http://vis.cs.brown.edu/results/bibtex/Abdul-2019-SVU.bib">(bibtex:
  Abdul-2019-SVU)</a>.