This paper explores the appropriateness of testing the equality of two means using either a t test, the Welch test, or the Wilcoxon-Mann-Whitney test for two independent samples based on the results of using two classes of preliminary tests (i.e., tests for population variance equality and symmetry in underlying distributions).
Perry, Kimberly T.
"A Critical Examination Of The Use Of Preliminary Tests In Two-Sample Tests Of Location,"
Journal of Modern Applied Statistical Methods: Vol. 2
, Article 5.
Available at: http://digitalcommons.wayne.edu/jmasm/vol2/iss2/5