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.

Ask again with the specifics and you will get a better answer than this one.

One thing that is still open after Dr.SurgeonPGH’s answer:

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

Dr.PeteFamMed 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…

There is a second half to this that has not been said yet. 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.

OP back with an update, since a thread like this is useless without one.

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.

traveltech_sara said:

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

Right, and one flattering data point from a group buy is not consistency. Consistency means separate batches, separately commissioned, over months.