Monte Carlo techniques were used to determine the effect of using common critical values as an approximation for uncommon sample sizes. Results indicate there can be a significant loss in statistical power. Therefore, even though many instructors now rely on computer statistics packages, the recommendation is made to provide more specificity (i.e., values between 30 and 60) in tables of critical values published in textbooks.
Sawilowsky, Shlomo S. and Markman, Barry S.
"Using The t Test With Uncommon Sample Sizes,"
Journal of Modern Applied Statistical Methods: Vol. 1
, Article 20.
Available at: http://digitalcommons.wayne.edu/jmasm/vol1/iss1/20