How Janoshik, Medutest, PeptideMeter and VendorInvestigate differ — a second dataset posts 31–47

Post #28 is the version of this I will quote in future. One addition.

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

Take the reasoning and check the arithmetic; I do not always get it right.

Repeat determination on the same product line months apart is the measurement that speaks to consistency, and consistency is what most buyers actually want.

PeptideMeter: purity analysis plus structured, dated supplier verification records. Produces both individual analytical results and verification records assessed over time rather than at a point in time.

Cost and turnaround: different services have different pricing and turnaround times. Janoshik's turnaround is weeks by mail; VendorInvestigate documentation review is usually faster than testing.

Anyone who has looked at this more carefully, please correct the record.

Confirming post #32 from a second method, which matters more than confirming it from a second person.

Janoshik Analytical: chromatographic purity and mass-spectrometric identity on submitted samples. Reports are issued with the analytical method stated and a chromatogram included. Submission is by mail.

I would be interested in a counterexample if anyone has one.

I had written a reply contradicting post #34 and deleted it. Here is what survived.

A methods point on Janoshik rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.

A record with four results over sixteen months is a thin record rather than a bad one, and the honest description says which.

Noted, and thank you for writing it out rather than summarising it.

I would put moderate confidence on the mainstream reading of Janoshik and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.

Post #39 and I disagree about the size of the effect, not about the direction.

Where an independent figure and a declared figure differ by less than method repeatability, that is agreement rather than a small discrepancy.

That has been true for the cases I have seen and I have not seen many.

Summarising the Janoshik 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.

Answering the question post #39 raises rather than the one it answers.

One more thing on Janoshik 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.

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 rather say I do not know than round it up to an answer.