The opening post is right about the mechanism and I think understates the practical bit. Since Designing a personal experiment keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.
Second pass at: Designing a personal experiment that could change your mind Writing it up because I had to work it out twice and would rather nobody else did.
The question about Designing a personal experiment that I actually want answered is the second one below. The first is context and I have kept it short.
Both are stated with units, and I have said what I already checked so that nobody repeats it.
Coming back to the opening post, because the follow-up matters more than the original answer.
The most useful reply I ever got about Designing a personal experiment was a request to state my units. It sounds like pedantry and it has saved me twice.
The opening post is right about the mechanism and I think understates the practical bit.
Since Designing a personal experiment keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.
Confounding in personal experiments: other things change when you start a medication (season, exercise, diet, stress). Documenting those confounders helps you understand their contribution to the result.
I read post #5 twice before replying, because I had assumed the opposite.
One more thing on Designing a personal experiment that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.
A rolling mean over several days is far more informative than any single reading for anything that varies day to day, which is nearly everything.
Adding it because I spent an afternoon working it out and nobody should have to twice.
Building on post #8 rather than restating it.
The honest answer on Designing a personal experiment is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
A single-person experiment can be genuinely informative if it has a pre-specified outcome, a defined period, and a plan written before the data arrives. Most accounts here have none of those.
I would be glad to be shown a cleaner way of putting this.
It does not tell you about anybody else, which is why aggregating these accounts does not produce evidence of the kind people want it to.
Nothing in a personal experiment is medical advice to anyone including yourself, and several people here have found that a prescriber will happily discuss a well-documented one.
The reasoning is more useful than the number, which is why I have shown it.
Narrowing post #14, because the general version has more than one answer.
The practical version of Designing a personal experiment is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
I would keep Designing a personal experiment and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
Generalisability: a robust n-of-1 result applies to you. It does not tell you much about whether the effect generalises to others similar to you, much less to people different from you.
That holds for the case as described. Change the assumptions and it may not.
Designing a personal experiment that could actually change your mind: that is the standard for an n-of-1 design. An experiment designed so that any result confirms what you already believed has not changed anything.
Post #21 answers the question as asked. The question underneath it is different.
Regression to the mean explains a great many personal experiments. People start when things are at their worst, and things at their worst tend to improve.
Genuinely open to being wrong about this one.
Record everything that changed, not just the intervention. Personal experiments are confounded by life, and the confounders are only recoverable if they were written down.
I would rather be precise about what I do not know than vague about what I do.
I have been on both sides of the Designing a personal experiment argument in this category within eighteen months, which should tell you how strong the evidence for either side is.
An n of one tells you about one person, which is the person you are most interested in. That is the whole value and it is not nothing.
One of those cases where knowing the mechanism does not help the decision.
Designing a personal experiment looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.