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

The arithmetic in post #28 is right; the assumption feeding it is the part to check.

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.

Answering the question post #30 raises rather than the one it answers.

Range and working range are different things and a certificate rarely distinguishes them. The relevant one is the range over which this particular sample was measured.

A guess, clearly labelled as one.

Reporting a result to more decimal places than the method's precision supports is a small dishonesty that appears everywhere. A method with a two per cent relative standard deviation does not support a figure quoted to a hundredth.

Second this, and I would have said it less carefully.

Narrowing post #32, because the general version has more than one answer.

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.

The confident version of this sentence would be wrong, so here is the hedged one.

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.

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).

That is clearer than the version I had in my head. Thank you.

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.

Narrowing post #43, because the general version has more than one answer.

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.