Red flags on COAs — my results so far

One concrete data point for the thread. The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as well as purity. A report failing any of those is weak evidence regardless of the percentage on it.

Worth separating that from certificates of analysis, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

A narrower follow-up, since the general answer is now clear:

How people are distinguishing a supplier having a bad batch from a supplier on the way out?

Dr.RaviCardio said:

The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as…

Adding the part of the answer the thread has not reached. Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself calibrated, the gradient and column, the detection wavelength, and where the integration baseline was drawn. A percentage point or two between competent labs is expected. What is not expected is a large gap, and the usual explanation there is that one report is not of the batch in your hand.

Closing the loop on my own question.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.

Dr.SleepRoch said:

Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…

Agreed, with a caveat about community reputation: it is a lagging indicator. Reports arrive weeks after orders, so a supplier can look excellent for a month after quality has already changed.