Pre-registering a personal experiment, seriously — setting out what I have, and where I think it stops being reliable.
A narrow question about pre-registering a personal experiment, deliberately narrow, because the broad version has been asked here four times and produced four long threads and no answer.
One question, stated units, stated method, and what I have already ruled out.
Write down what you would accept as evidence that it did not work, before starting. That one sentence turns an anecdote into an experiment.
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.
Not the whole picture, but the part of it I can speak to.
Coming back to post #3, because the follow-up matters more than the original answer.
My experience of pre-registering a personal experiment contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Nobody has said the unglamorous part of pre-registering a personal experiment yet, so: most of the variation is explained by things that are boring to write about and easy to check.
Sample size in n-of-1: you are the sample. Repeated measurements (weekly weighings, daily mood scores) increase the power to detect a real effect even though n=1.
Adding the caveat now so it does not have to be extracted later.
I have no financial interest in anything named in this thread and I want to say so before I comment on pre-registering a personal experiment, because it is the sort of subject where it matters.
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.
I keep a log of this specifically because memory is unreliable about it.
Post #11 is right about the mechanism and I think understates the practical bit.
Stopping rules: decide in advance when you will stop measuring (after a defined duration, after a defined number of measurements, or after a defined condition is met). Not deciding in advance means stopping when the result satisfies you, which is bias.
Worth separating two things that post #10 runs together.
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.
That is one dataset and I would not build a rule on it.
Measure the same thing the same way at the same time of day. Most of the noise in personal data is measurement protocol rather than biology.
I would be interested in a counterexample if anyone has one.
My position on pre-registering a personal experiment is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
On post #14 — agreed on the reasoning, with one qualification.
Multiple outcomes measured without a primary one means something will move. Nominate the primary in advance and report the rest as secondary.
This follows post #16 rather than contradicting it.
The bit of pre-registering a personal experiment that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
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.
Marking that as an opinion rather than a finding.
I read post #19 twice before replying, because I had assumed the opposite.
I changed my mind about pre-registering a personal experiment after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
Two claims get bundled together under pre-registering a personal experiment and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
A photograph or a device reading is a harder record than a recollection, and where one is available it should be the record.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Coming back to post #23, because the follow-up matters more than the original answer.
The most useful thing anyone has posted about pre-registering a personal experiment in this category was a table of what had been measured and by whom. That is what I would want again.
No disagreement from me. Posting only so the question does not look ignored.
Reporting the whole series rather than the interesting segment is the discipline that makes personal data worth reading. Selective reporting is the default without effort.
I have deliberately not rounded that, because the rounding is where the argument starts.
Duration should be set by how long an effect would take to appear rather than by patience. Stopping early is the commonest reason a personal experiment answers nothing.
I would treat the number as indicative rather than as a measurement.
This follows post #28 rather than contradicting it.
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.
I have separated what I observed from what I concluded, which does not always happen.