What even IS a COA and why should I care — my results so far

RunnerRach said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

RunnerRach has the substance of this right. The condition it depends on is worth stating. The provenance questions are more useful than the numbers. Does the report name the batch on your vial. Is the date consistent with when that batch existed. Is it addressed to the vendor or to the person who paid for the test. Was the sample taken from general stock or supplied by the vendor. A perfect certificate for a different batch is worth nothing, and vendor-supplied samples are the most common way a genuine report ends up misleading.

RunnerRach said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

I am more sceptical of certificates as a category than this thread is. The document tells you about a sample somebody chose to send, at a moment in time, and nothing about the vial in your hand. Independent purchase-and-test is a different quality of evidence and everything else is inference.

Taking the question as asked, rather than the general version of it. 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.

Dr.ObesityMed said:

The provenance questions are more useful than the numbers.

Adding a me-too, because a thread of one person's experience is not much use. I had assumed I was the exception until I read this.