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? Two numbers on the same units: do they move together?
Is the relationship roughly a straight line, without strong outliers? Yes: roughly a straight line
The test
Pearson correlation test
Assumes
A straight-line relationship; roughly normal; no strong outliers.
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.