Timeline for Normality, and when to use t-test vs. Mann-Whitney U-test?
Current License: CC BY-SA 3.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| S Aug 2, 2018 at 0:10 | history | mod moved comments to chat | |||
| S Aug 2, 2018 at 0:10 | comment | added | Glen_b | @Alexis ... I have moved the conversation to chat. | |
| Oct 12, 2017 at 20:53 | comment | added | Alexis | Updated links: One can use the Shapiro-Wilk test to test for equivalence to normality, rather than difference from normality using the tost package for Stata. Doing so and combining with the standard test for different from normality will permit one to distinguish between trivial and relevant difference from normality, as well as to conclude equivalence to normality. | |
| May 11, 2015 at 2:07 | comment | added | Glen_b | If those plots are typical of your distribution shapes, it will matter little which test you choose. You have mild skewness, it won't harm the properties of the t very much (though I'd lean toward MW or a permutation test of means myself). If you use a $t$, make sure you allow for unequal variance (that is, do Welch or something along those lines --- and don't test for equal variances, just assume they're different). | |
| May 11, 2015 at 1:59 | comment | added | Glen_b | The idea that you can have "a hard and fast normal vs non-normal" is illusory. If you aren't in a position to say (e.g. from some a priori knowledge of the distribution shape, the sample size or both) "the t-test should be fine", you should not assume it. Deciding which test to do on the basis of the sample leaves neither test having its nominal properties, and the resulting overall properties are often substantially less useful than simply doing Mann-Whitney; if you have reason (a priori) to think that the tails are not too heavy & distribution not very skew, the t-test should be fine | |
| May 10, 2015 at 17:21 | vote | accept | user2416002 | ||
| May 10, 2015 at 17:18 | history | edited | gung - Reinstate Monica | CC BY-SA 3.0 |
changed tag; light editing
|
| May 10, 2015 at 17:17 | answer | added | gung - Reinstate Monica | timeline score: 8 | |
| May 10, 2015 at 16:48 | comment | added | user2416002 | Thank you for the link above - very good read. I'm using SPSS, but I am not 100% sure how the software is choosing the line. | |
| May 10, 2015 at 16:34 | comment | added | gung - Reinstate Monica | Do you know how the reference lines were drawn in your software? Usually the line goes through the 25th & 75th percentile, but that doesn't seem to be the case here. | |
| May 10, 2015 at 16:32 | comment | added | gung - Reinstate Monica | Closely related: How to choose between t-test or non-parametric test e.g. Wilcoxon in small samples? | |
| May 10, 2015 at 16:29 | history | asked | user2416002 | CC BY-SA 3.0 |