To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
If the populations have different shapes or different standard deviations, use a 2-Sample t without pooling variances.
However, if you have more than 15 observations in each sample or your data aren't severely skewed, use a 2-Sample t because the test has more power.
In statistics, random samples are used to make generalizations, or inferences, about a population. If your data are not collected randomly, your results may not represent the population. For more information, go to Randomness in samples of data.