Ultrasound and skinfolds: operator dependence in practice posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Reading back through the ultrasound and skinfolds threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
Ultrasound and skinfolds is a good example of a question where the honest answer is boring and the interesting answers are unsupported. I would go with boring.
The thing about ultrasound and skinfolds that took me longest to accept is that a plausible mechanism is not evidence of an effect. It is a reason to look, not a result.
Percentage body fat is a ratio, so it changes when either component changes. A rising percentage during weight loss is arithmetic before it is physiology.
Small correction to my own earlier position on ultrasound and skinfolds. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
The question underneath ultrasound and skinfolds is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
The claim is narrower than it sounds, and deliberately so.
A request rather than an answer: could whoever has the primary source for ultrasound and skinfolds post it? I have seen the claim three times this month and each version had lost a qualifier.
Where I part company with post #41, and it is a narrow parting.
Ultrasound and skinfolds is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.
Collapsed as off-topic by two members at trust level 3 or above
Post #41 is the version of this I will quote in future. One addition.
Percentage body fat is a ratio, so it changes when either component changes. A rising percentage during weight loss is arithmetic before it is physiology.
The arithmetic in post #45 is right; the assumption feeding it is the part to check.
I changed my mind about ultrasound and skinfolds 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.
Helpful, and short, which on this subject is harder than long.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.
Collapsed as off-topic by two members at trust level 3 or above
Everything in post #45 holds. The case it does not cover is the one I have.
My position on ultrasound and skinfolds is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
Narrowing post #48, because the general version has more than one answer.
Worth stating the null on ultrasound and skinfolds before we explain it: the observation may be nothing. That possibility deserves a sentence and usually does not get one.
Confirming post #51 from a second method, which matters more than confirming it from a second person.
The practical version of ultrasound and skinfolds is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
The failure mode on ultrasound and skinfolds is boring rather than dramatic. It is almost always the step everyone assumes was done correctly because it is too simple to get wrong.
Since ultrasound and skinfolds 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.
Post #56 is the version of this I will quote in future. One addition.
One more thing on ultrasound and skinfolds 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.
Where I part company with post #54, and it is a narrow parting.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.