Suppose that you want to perform a hypothesis test to compare the means of two populations using independent simple random samples. Suppose that preliminary data analyses of the samples suggest that the distribution of the variable under consideration is skewed to the left for one population and skewed to the right for the other. The sample sizes are 50 and 60. Should you use the pooled t-test, the nonpooled t-test, the Mann-Whitney test, or none of these tests?