What I would check first on purity-only is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
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Sourcing · Independent verification · continued
[2026 update] What a purity-only report leaves out posts 61–90
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
t.nardone, post #25: Post #23 describes the usual case. This is about the unusual one. Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want. Go to post
Useful. I had the fact and not the reason, which turns out to be the important half.
Collapsed as off-topic by two members at trust level 3 or above
A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which.
compounding_ruth, post #34: Post #32 and I disagree about the size of the effect, not about the direction. A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which. Filing this under things that are true until someone shows me otherwise. Go to post
Two sentences on purity-only and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
a.adebayo, post #20: On post #18 — agreed on the reasoning, with one qualification. The question underneath purity-only 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,… Go to post
Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want.
Post #64 and I disagree about the size of the effect, not about the direction.
Purity-only: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.
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Repeat testing: if you want to verify the same lot a second time, testing with a different service strengthens the conclusion. Testing with the same service verifies reproducibility but uses the same method.
I would put the burden of proof on the interesting explanation, not the dull one.
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Helpful, and short, which on this subject is harder than long.
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The version of purity-only that I was taught turned out to be a teaching simplification. Useful, and not true in the way I had assumed it was.
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Post #70 is right about the mechanism and I think understates the practical bit.
The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible.
Speaking for myself and not for anyone else who has posted here.
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This settles it for me, at least until somebody posts a reason it should not.
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Wickramasinghe, post #15: Answering the purity-only question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken. Go to post
A note on scope: what I am saying about purity-only applies to the case in the first post and I would not extend it further without checking.
t.vasquez, post #32: Adding a small correction to the purity-only summary above rather than a disagreement with it. The substance holds; one of the figures is out by a factor that matters. Go to post
On post #73 — agreed on the reasoning, with one qualification.
Purity-only has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.
Post #74 answers the question as asked. The question underneath it is different.
It does not extend to lots nobody has seen, it is not a recommendation, and it says nothing about anything other than what was measured.
Noting that I have skin in this question and have tried to discount for it.
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I read post #73 twice before replying, because I had assumed the opposite.
Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want.
A weak preference rather than a position.
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t.nardone, post #25: Post #23 describes the usual case. This is about the unusual one. Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want. Go to post
Adding a null result on purity-only. I looked, carefully, and found nothing, and null results deserve posting precisely because they never are.
KStephanopoulos, post #61: What I would check first on purity-only is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph. Go to post
A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which.
Not a strong opinion, just a consistent one.
Nothing to add on the substance. Thank you for taking the question at face value.
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l.vukovic, post #4: Post #3 is right about the mechanism and I think understates the practical bit. It does not extend to lots nobody has seen, it is not a recommendation, and it says nothing about anything other than what was measured. A single observation, in a thread that deserves better than single observations. Go to post
An update on my earlier purity-only post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Purity-only sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.
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Everything in post #81 holds. The case it does not cover is the one I have.
Repeat testing: if you want to verify the same lot a second time, testing with a different service strengthens the conclusion. Testing with the same service verifies reproducibility but uses the same method.
Noting that the question and the thing people usually mean by it are different.
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Narrowing post #85, because the general version has more than one answer.
The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible.
Two people can read the same figure differently here and both be reasonable.
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i.bakken, post #81: I read post #77 twice before replying, because I had assumed the opposite. The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible. On balance I think that is right, and I would not bet much on it. Go to post
I think the purity-only question is answerable and has not been answered, which is a more optimistic position than most of this thread.
h.castellanos, post #62: Useful. I had the fact and not the reason, which turns out to be the important half. Go to post
What I would tell a new member reading about purity-only for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
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A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which.
The number is defensible. The precision I gave it is not.