Which test? The test chooser
Questions run from left to right; the path lights up as you answer, and each leaf on the right is a test. On a phone, the tree scrolls sideways in its box.
The questions
What kind of outcome did you measure? A number: a measurement such as days, millilitres or euros
How many groups are you comparing? Two groups
Are the two groups independent or paired? Independent: different units in each group
What must the test show? That the means differ
Welch's two-sample t test
- Assumes
- Independent groups and observations; each group's mean roughly normal. Equal variances are not needed.
- How to check
- Plot both groups; Levene's test only as information, never to choose the test.
- If it fails
- Mann–Whitney or a permutation test (chapter 14).
- Python (scipy, statsmodels)
ttest_ind(a, b, equal_var=False)- Excel
=T.TEST(a, b, 2, 3)
Which test fits depends on what you measured, how many groups you compare, whether the groups are independent or paired, and whether σ is known. Answer one question at a time: the path lights up in the tree, and the leaf names the test with its assumptions, what to do if they fail, and the code. Or switch to the drill and name the test for a scenario yourself.