Specificity, linearity, accuracy, precision, range, robustness — with real criteria

The opening post is the version of this I will quote in future. One addition.

My position on specificity is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.

Post #5 and I disagree about the size of the effect, not about the direction.

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.

That distinction has done more work for me than anything else in this category.

This is the first time the answer has come with its own limits attached. Appreciated.

The honest answer on specificity is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.

Most people get the first two right and then argue about the fourth.

A method that reports the same number for every lot is worth a second look. Real processes vary, and a total absence of variation is a statement about the measurement rather than the process.

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: the detector response is proportional to compound concentration across the working range. Demonstrated by running standards at multiple concentrations and showing R-squared values typically ≥0.99.