Specificity, linearity, accuracy, precision, range, robustness — with real criteria posts 31–49

This follows post #28 rather than contradicting it.

Where the specificity reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.

I read post #30 twice before replying, because I had assumed the opposite.

Linearity across the working range is a routine demonstration and it constrains how far a result can be extrapolated. A method linear from 80 to 120 per cent of nominal says nothing about a sample at ten per cent.

A guess, clearly labelled as one.

Robustness testing deliberately varies the parameters most likely to drift — organic percentage, pH, temperature, flow — and shows the result does not. It is the part of validation that predicts whether a method will transfer.

Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.

Following this. I have the same question and no better information than the first post.

Forced degradation under acid, base, oxidation, heat and light is how specificity is actually demonstrated. The chromatograms from that work are more informative than the release data.

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.

The strongest argument against my own position on specificity, stated as well as I can state it, since nobody else has yet.

Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.

System suitability: injections run at the start of a batch to establish that the instrument and column are performing. Acceptance criteria typically include replicate precision (RSD ≤2%), peak tailing (0.8–1.5), theoretical plates (>2000), and resolution (>1.5).

Bookmarking this. I will come back when I have something worth adding.

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.

Post #43 answers the question as asked. The question underneath it is different.

Having read the whole specificity thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.