Repeatability versus intermediate precision posts 61–90
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
Confirming post #59 from a second method, which matters more than confirming it from a second person.
Answering the repeatability versus intermediate precision question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.
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.
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I would put moderate confidence on the mainstream reading of repeatability versus intermediate precision and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.
Small correction to my own earlier position on repeatability versus intermediate precision. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
Post #63 describes the usual case. This is about the unusual one.
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 claim is narrower than it sounds, and deliberately so.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
Post #67 put the caveat in the right place and I want to underline it.
The confident answers on repeatability versus intermediate precision and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
Post #70 is the version of this I will quote in future. One addition.
The arithmetic on repeatability versus intermediate precision is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
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Where the repeatability versus intermediate precision discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.
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.
If anyone has run this properly I would rather read that than my own guess.
Answering the question post #73 raises rather than the one it answers.
Offering a way to settle repeatability versus intermediate precision rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
Confirming post #74 from a second method, which matters more than confirming it from a second person.
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.
I have written this out at length because the short version keeps being misread.
I had written a reply contradicting post #73 and deleted it. Here is what survived.
A note on how repeatability versus intermediate precision gets discussed rather than on repeatability versus intermediate precision itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
My position on repeatability versus intermediate precision is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
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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.
Post #76 and I disagree about the size of the effect, not about the direction.
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.
Two people can read the same figure differently here and both be reasonable.
Worth separating two things that post #77 runs together.
The documentation on repeatability versus intermediate precision is better than this thread and I say that as someone who has posted in the thread.
This follows post #81 rather than contradicting it.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
That is the version I would defend. It is not the version I started with.
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.
That is clearer than the version I had in my head. Thank you.
Repeatability versus intermediate precision: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.
Taking post #85 at face value and following it one step further.
Precision has two levels worth distinguishing: repeatability within a run and intermediate precision across days, analysts and instruments. The second is where most methods lose the numbers people quote.
A qualification I should have led with rather than closed on.
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I read post #85 twice before replying, because I had assumed the opposite.
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.
The rule of thumb is fine; the edge cases are where it earns its keep.
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).
Repeatability versus intermediate precision was covered in the wiki last year and the page has a review date on it, which is a better starting point than my memory of a thread.