Interpreting a composition change that is within the error bars

On the subject in the title: Interpreting a composition change that is within the error bars Working notes rather than a conclusion.

Asking about interpreting a composition change directly, because I have read four threads on it and each answered a slightly different question.

The version I want answered is the narrow one: given the method stated below, is the result within what anyone else has seen? I am not asking what it means yet.

Method, numbers and the two assumptions I am aware of making are below. If the assumptions are wrong that is more useful to me than agreement.

DXA (dual-energy X-ray absorptiometry): the standard for body composition in research. Precision is roughly ±1 kg for lean mass. Two scans six months apart on the same machine with the same technician give you useful information. Two scans six weeks apart on different machines tell you about the machines.

Two people can read the same figure differently here and both be reasonable.

Picking up post #2: that is the part I would want checked first.

The most useful reply I ever got about interpreting a composition change was a request to state my units. It sounds like pedantry and it has saved me twice.

On the opening post — agreed on the reasoning, with one qualification.

Counterpoint on interpreting a composition change, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.

Post #2 is the version of this I will quote in future. One addition.

Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.

Helpful, and short, which on this subject is harder than long.

Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.

On balance I think that is right, and I would not bet much on it.

Post #4 describes the usual case. This is about the unusual one.

Summarising the interpreting a composition change thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.

The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.

I read post #8 twice before replying, because I had assumed the opposite.

Interpreting a composition change came up in a thread eighteen months ago and was answered well. I cannot find it, which is itself the problem, so here is the reconstruction.

A single scan is a snapshot with error bars nobody prints. Two scans give you a difference with larger error bars.

I would want a second opinion before relying on that.

Comparing a body composition result against a population reference is a different exercise from tracking your own change, and the second is what most people want.

Someone will know this better than I do and I hope they say so.

Practical experience of interpreting a composition change, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.

Narrowing post #11, because the general version has more than one answer.

Precision versus accuracy: the precision you need depends on what question you are answering. "Am I losing fat?" requires less precision than "Is my lean mass preservation within the expected range?"