On the subject in the title: Specificity, linearity, accuracy, precision, range, robustness — with real criteria — a second dataset Working notes rather than a conclusion.
Posting a small dataset on Specificity. It is mine, it is uncontrolled, and the method is stated so it can be discounted appropriately.
What I would like is not agreement but a second dataset collected by someone with no stake in mine. If one exists I would rather read it than argue for this one.
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.
Not a strong opinion, just a consistent one.
Right — I had this wrong and I am glad to have read it before it mattered.
That is a fair summary of where the discussion has got to.
Post #6 is right about the mechanism and I think understates the practical bit.
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.
If anyone can point at the primary source I would be grateful.
Worth separating two things that post #6 runs together.
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.
None of the above is medical advice and I am not qualified to give any.
Robustness: the method gives consistent results when minor parameters vary. Tested by deliberately varying pH, temperature, flow rate, and mobile phase composition within reasonable ranges and demonstrating that results stay within acceptance.
Coming back to post #8, because the follow-up matters more than the original answer.
What I would tell a new member reading about Specificity for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
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.
If it helps: the failure mode here is usually boring rather than dramatic.
Helpful, and easy to find again, which is half of what a good reply is.
An independent laboratory's method being different from the supplier's is not a discrepancy. It becomes one only when the results differ by more than both methods' demonstrated precision.
I read post #16 twice before replying, because I had assumed the opposite.
Two things can be true about Specificity at once: the mechanism is plausible and the evidence for the size of the effect is thin. Most of the argument here is people defending the first against attacks on the second.
This follows post #16 rather than contradicting it.
I keep a log for Specificity specifically because my memory of it turned out to be systematically wrong in one direction. Six weeks of notes cost nothing and settled it.
On post #16 — agreed on the reasoning, with one qualification.
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.
Correct me on the arithmetic if it is wrong; I would rather know.
Specificity is the first question: does the method separate the analyte from everything reasonably expected to be present? A method that has not been challenged with its own degradation products has not answered it.
I would call that likely rather than established.
On Specificity the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
This follows post #20 rather than contradicting it.
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 all the detail I have. Someone else will have more.
Seconded. It reads as careful rather than confident, which is the right register.
Posting my Specificity numbers with the method attached so they can be discounted properly. Uncontrolled, unblinded, and collected by someone who wanted a particular answer.
Specificity: the method can distinguish the intended compound from related impurities and degradation products. Tested by comparing results on pure compounds, mixtures of compounds, and degraded samples.
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.
Worth saying I have only my own numbers here, and n is small.
Worth separating two things that post #26 runs together.
Precision and repeatability: within-run and between-run variability of the method. Acceptance criterion is typically a relative standard deviation of ≤2% for area measurements.
I keep a log of this specifically because memory is unreliable about it.