Specificity, linearity, accuracy, precision, range, robustness — with real criteria Writing it up because I had to work it out twice and would rather nobody else did.
Trying to establish the boundary conditions on specificity. The general claim is fine; I want to know where it stops holding, because that is where I am operating.
Three edge cases below, in increasing order of how far outside the usual range they sit.
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.
If you are new and reading this thread for the answer to specificity: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping.
Adding thanks rather than a view. I do not have a view worth the space.
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.
The strength of my opinion here exceeds the strength of my evidence.
Everything in post #5 holds. The case it does not cover is the one I have.
Accuracy: the method measures what you intend to measure. For purity methods, this is tested by spike-and-recover experiments: add a known amount of impurity to a sample and measure whether you recover the added amount.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
The bit of specificity that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
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.
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.
The disagreement above is smaller than it looks once the terms are fixed.
Post #10 answers the question as asked. The question underneath it is different.
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.
Filing this under things that are true until someone shows me otherwise.
This is the first time the answer has come with its own limits attached. Appreciated.
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.
Worth separating two things that post #13 runs together.
Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
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.
The part I am sure of is shorter than the part I have written.
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.
If this contradicts something upthread, the upthread version may well be the better one.
Post #16 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.
Genuinely open to being wrong about this one.
Distinguishing three things in the specificity discussion that keep getting used interchangeably: the observation, the proposed mechanism, and the recommendation that gets attached to both.
Staff rationale: Hidden by community flags. The claim about a named supplier was not accompanied by a batch, a date, a method or a document, which R6 requires.
On post #17 — agreed on the reasoning, with one qualification.
Accuracy for a purity assay is usually demonstrated by spike recovery across a range. Without a reference standard, that is exactly the work that cannot be done, which is why reference material matters.
The corresponding entry is in the public moderation log. Hidden posts are never deleted.
Picking up post #20: that is the part I would want checked first.
The most useful reply I ever got about specificity was a request to state my units. It sounds like pedantry and it has saved me twice.
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.
I have left out the parts I could not verify.
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.
It reads as pedantry until the day it does not.
Adding the measurement that post #25 says would settle it.
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.
Where I part company with post #25, and it is a narrow parting.
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.
Scoping that to what I have actually seen rather than what I have read.
I read the earlier replies on specificity twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.
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.