Posting this under the heading it deserves: Splitting a vial for two laboratories: how to do it without ruining both — what changed since Everything below is what sits behind that.
Something about Splitting a vial does not reconcile and I would like a second pair of eyes before I decide which half is wrong.
Two sources, both reputable, giving figures that cannot both be right unless they are measuring different quantities. My suspicion is that they are, and I cannot see how.
I had written a reply contradicting the opening post and deleted it. Here is what survived.
Where the Splitting a vial reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
How to submit a sample: contact the service directly. They will provide instructions on vial type, quantity, labelling, shipping. Following their instructions exactly ensures results are interpretable.
Filing this under things that are true until someone shows me otherwise.
Adding a null result on Splitting a vial. I looked, carefully, and found nothing, and null results deserve posting precisely because they never are.
Post #4 is the version of this I will quote in future. One addition.
The strongest argument against my own position on Splitting a vial, stated as well as I can state it, since nobody else has yet.
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.
Written quickly, so the reasoning may be tighter than the wording.
Combining a chromatographic result with a documentation review gives a more complete picture than two chromatographic results, because the failure modes are different.
Understood, and I withdraw the assumption I opened with.
Cross-checking one lot with two services independently is the cleanest test available and is rarely done because it costs twice as much.
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.
A single observation, in a thread that deserves better than single observations.
Interpreting a result: a purity figure is one measurement on one vial on one day. Agreement across two services is much stronger. A single high number does not establish a pattern.
I would be glad to be shown a cleaner way of putting this.
Post #10 put the caveat in the right place and I want to underline it.
Nobody has said the unglamorous part of Splitting a vial yet, so: most of the variation is explained by things that are boring to write about and easy to check.
Building on post #12 rather than restating it.
Answering the Splitting a vial 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.
Agreed, and I will stop repeating the version of this I had been repeating.
A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which.
Post #14 and I disagree about the size of the effect, not about the direction.
The question underneath Splitting a vial 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.
Small correction to my own earlier position on Splitting a vial. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
Adding the measurement that post #17 says would settle it.
The reason Splitting a vial keeps being re-asked is that the answer is conditional and people quote it without the condition. It is not that the answer is unknown.
Coming back to post #20, because the follow-up matters more than the original answer.
The record shows what was measured. Everything beyond that is inference and this site tries to keep the line visible.
If that reads as pedantic, it is, and it has saved me twice.
This follows post #22 rather than contradicting it.
The four services answer different questions. A documentation review cannot tell you what is in a vial and a purity assay cannot tell you whether the records are internally consistent.
Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want.
Confirming post #22 from a second method, which matters more than confirming it from a second person.
Whatever the answer on Splitting a vial turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.
That is clearer than the version I had in my head. Thank you.
Nothing in a verification record is a guarantee about the vial in your hand. That check is yours and nobody can do it for you.
The evidence for this is thinner than the way I have phrased it suggests.
Verification records are dated because they expire in usefulness. A result from two years ago describes a lot from two years ago.