[2026 update] What a purity-only report leaves out

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.

Nobody has said the unglamorous part of purity-only yet, so: most of the variation is explained by things that are boring to write about and easy to check.

I had written a reply contradicting post #14 and deleted it. Here is what survived.

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.

This is where my knowledge stops and I would rather mark the edge than blur it.

Small correction to my own earlier position on purity-only. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.

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, collecting it will not help.

Answering the question post #19 raises rather than the one it answers.

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 partial answer, offered because a partial answer beats none.

The arithmetic in post #19 is right; the assumption feeding it is the part to check.

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

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

Where the purity-only discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.

The arithmetic on purity-only is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.

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.

I have three months of notes on purity-only and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.

The bit of purity-only that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.

Fair, and the limits you put on it are the part I will remember.

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

The honest answer on purity-only is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.

Most people get the first two right and then argue about the fourth.