Advice: Unpaired t Test
Small to medium samples
UNPAIRED (TWO SAMPLE) STUDENT t TEST FOR THE COMPARISON OF MEANS: Listed under parametric methods section of the analysis menu. With small samples we must assume that the observations have been drawn from a normal distribution. The Shapiro-Wilk W test for non-normality can be used to test this assumption but if there is any doubt then you are strongly advised to use the distribution free (nonparametric) method of Mann-Whitney (Wilcoxon rank sum). You can sometimes use transformations (e.g. natural logs) to "normalise" your data then apply parametric methods to them but you are advised to seek statistical advice before doing this.