The decision tree: every question, every answer, every test
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? Paired: the same units measured twice, or matched pairs
The test
Paired t test
Assumes
Pairs independent of each other; the differences roughly normal (not the raw values).
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.