This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
Limits of detection and quantitation: LOD is the lowest concentration that produces a signal above background. LOQ is the lowest concentration at which the method meets precision and accuracy acceptance criteria. Both are determined empirically.
It reads as pedantry until the day it does not.
Taking post #120 at face value and following it one step further.
Taking repeatability versus intermediate precision seriously for a moment rather than deflecting: the honest position is that the community has observations and no controlled comparison, and those two things support very different sentences.
Marking my uncertainty on repeatability versus intermediate precision explicitly. I am confident about the direction, much less confident about the size, and not confident at all that it generalises past the case in the first post.
Post #124 answers the question as asked. The question underneath it is different.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
This has been discussed before and I could not find the thread, so, again.
I read post #123 twice before replying, because I had assumed the opposite.
My experience of repeatability versus intermediate precision contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Reading back through the repeatability versus intermediate precision threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
That is the distinction I keep failing to hold on to. Written down now.
On repeatability versus intermediate precision: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
Post #130 and I disagree about the size of the effect, not about the direction.
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.
I would be interested in a counterexample if anyone has one.
Source for the repeatability versus intermediate precision figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.
Repeatability versus intermediate precision is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.
System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.
Post #133 put the caveat in the right place and I want to underline it.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
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