Two unpaired t tests. When you choose to compare the means of two nonpaired groups with a t test, you have two choices: •Use the standard unpaired t test. It assumes that both groups of data are sampled from Gaussian populations with the same standard deviation. •Use the unequal variance t test, also called the Welch t test.. The Welch t Test is also known an Unequal Variance t Test or Separate Variances t Test. No outliers; Note:. Note that this form of the independent samples t test statistic assumes equal variances. Because we assume equal population variances, it is OK to "pool" the sample variances (s p). However, if this assumption is violated, the pooled.

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Many statistical procedures, such as analysis of variance (ANOVA) and regression, assume that although different samples can come from populations with different means, they have the same variance. Because the susceptibility of different procedures to unequal variances varies greatly, so does the need to do a test for equal variances.. The t test assumes your data: are independent; are (approximately) normally distributed; have a similar amount of variance within each group being compared (a.k.a. homogeneity of variance) If your data do not fit these assumptions, you can try a nonparametric alternative to the t test, such as the Wilcoxon Signed-Rank test for data with unequal.