This Monte Carlo study shows that the separate-variances Welch t test has inflated Type I error rates at very small sample sizes, especially when sample sizes are very small in one group and larger in the second group – even when all assumptions for the statistical test are met.
Adusah, Albert K. and Brooks, Gordon P.
"Type I Error Inflation of the Separate-Variances Welch t test with Very Small Sample Sizes when Assumptions Are Met,"
Journal of Modern Applied Statistical Methods: Vol. 10
, Article 33.
Available at: http://digitalcommons.wayne.edu/jmasm/vol10/iss1/33