Data Analysis with R, Second Edition - Second Edition

Assumptions of the independent samples t-test

Homogeneity of variance (or homoscedasticity, a scary sounding word), in this case, simply means that the variance in the miles per gallon of the automatic cars is the same as the variance in miles per gallon of the manual cars. In reality, this assumption can be violated as long as you use a Welch's T-test like we did, instead of the Student's T-test. You can still use the Student's T-test with the t.test function, like by specifying the optional parameter var.equal=TRUE. You can test for this formally using var.test or leveneTest from the car package. If you are sure that the assumption of homoscedasticity is not violated, you may want to do this because it is a more powerful test (fewer Type ...