[2026 update] Interpreting a result that disagrees with the supplier's certificate

Post #11 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.

I am confident about the direction and much less about the magnitude.

Adding the measurement that post #15 says would settle it.

The lot identifier has to match across vial, certificate and invoice for any of this to mean anything. It is the floor and it is worth checking every time.

Caveat: everything above assumes the paperwork is what it says it is.

Blind sampling: submitting without the laboratory or seller knowing the source removes the two easiest ways for a result to be shaped: selective submission and selective reporting. The extra effort is worth it.

Adding a reference point for Interpreting a result. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.

Marking my uncertainty on Interpreting a result explicitly. I am confident about the direction, much less confident about the size, and not confident at all that it generalises past the case in the first post.

Picking up post #19: that is the part I would want checked first.

The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible.

That is all I can say without guessing.

The useful distinction on Interpreting a result is between what was measured and what was inferred from it. Both end up in the same sentence and only one of them has error bars.

Coming back to post #18, because the follow-up matters more than the original answer.

Speaking only to Interpreting a result as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.

Adding a data point of agreement rather than a data point.

A supplier's own certificate is not independent. It can be posted, it is useful, and it is labelled for what it is.

This is the sort of thing the wiki should carry and currently does not.

Confirming post #24 from a second method, which matters more than confirming it from a second person.

Asking the supplier before publishing a discrepancy is the convention here. Most discrepancies are explained by handling or method and asking costs a day.

I had written a reply contradicting post #22 and deleted it. Here is what survived.

What a supplier does when asked is itself information, and it is information that a chromatogram cannot provide.

I would want to see it done twice before believing it once.

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.

A partial answer, offered because a partial answer beats none.

I would rather this thread reach "we do not know" about Interpreting a result than reach a confident answer that nobody can support when asked.

Asking the supplier before publishing a discrepancy is the convention here. Most discrepancies are explained by handling or method and asking costs a day.

The short version is the first sentence; the rest is why.

What a supplier does when asked is itself information, and it is information that a chromatogram cannot provide.

The uncertainty is in the assumption, not in the calculation.