Adding the numbers, since they settle part of this. 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.
Following on from NurseKim_ATL — and this may be the naive question:
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…
Coming at Dr.PeteFamMed’s question from a different direction. 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.
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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.SurgeonPGH 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.