Previous research suggests that the power of the independent-samples t test decreases when population distributions are mixed normal rather than normal, and that robust methods have superior power under these conditions. However, under some conditions, the power for the independent-samples t test can be greater when the population distributions for the independent groups are mixed normal rather than normal. The implications of these results are discussed.
Thompson, Marilyn S.; Green, Samuel B.; Chen, Yi-hsin; Stockford, Shawn; and Lo, Wen-juo
"Power of the t Test for Normal and Mixed Normal Distributions,"
Journal of Modern Applied Statistical Methods:
2, Article 25.
Available at: http://digitalcommons.wayne.edu/jmasm/vol4/iss2/25