rachel_ABQ said:

The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either.

Coming at rachel_ABQ’s question from a different direction. It helps to say which part of this you are uncertain about. A precise question gets a precise answer; a general one gets everybody’s favourite anecdote.

That is the short version; the long version is somebody else's post.

The figures, for anyone assembling their own picture. One practical note: write down what you did and when, before you need it. Reconstructing a timeline from memory three months later is how people end up unable to answer the one question that would have resolved it.

One thing that is still open after rachel_ABQ’s answer:

How long did you give it before you decided it was working?

Found this through search years later and it is still the best discussion of the question, so adding one thing. If you are comparing against somebody else’s result, check that you are comparing the same measurement taken the same way. Most of the apparent contradictions in these threads dissolve at that step.

NicoleRaleigh said:

One practical note: write down what you did and when, before you need it.

No disagreement, though it depends on where somebody is starting from, and this thread has been assuming a starting point nobody stated.