The opening post put the caveat in the right place and I want to underline it. Agreed on repeatability versus intermediate precision, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part…
Repeatability versus intermediate precision — setting out what I have, and where I think it stops being reliable.
Trying to establish the boundary conditions on repeatability versus intermediate precision. 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.
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.
The disagreement above is smaller than it looks once the terms are fixed.
The opening post put the caveat in the right place and I want to underline it.
Agreed on repeatability versus intermediate precision, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.
Building on post #3 rather than restating it.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
Practical answer on repeatability versus intermediate precision, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
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.
I am confident about the direction and much less about the magnitude.
Post #7 is the version of this I will quote in future. One addition.
The version of repeatability versus intermediate precision that circulates here is a simplification of a simplification. It is not wrong, but it has lost the conditions under which it holds, and those conditions are where the interesting cases live.
Adding a data point of agreement rather than a data point.
Two claims get bundled together under repeatability versus intermediate precision and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
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).
This follows post #12 rather than contradicting it.
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.
Worth separating two things that post #11 runs together.
On repeatability versus intermediate precision, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
I read post #14 twice before replying, because I had assumed the opposite.
The most useful thing anyone has posted about repeatability versus intermediate precision in this category was a table of what had been measured and by whom. That is what I would want again.
Taking post #16 at face value and following it one step further.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
I have deliberately not rounded that, because the rounding is where the argument starts.
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.
This is the sort of thing the wiki should carry and currently does not.
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.
I would put the burden of proof on the interesting explanation, not the dull one.
Narrowing post #19, because the general version has more than one answer.
Two people in this thread mean different things by repeatability versus intermediate precision and are disagreeing about the definition while believing they are disagreeing about the facts. Worth pausing to define it.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
Answering the question post #23 raises rather than the one it answers.
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.
Worth reading the earlier posts in this thread before acting on mine.
The arithmetic in post #23 is right; the assumption feeding it is the part to check.
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 one dataset and I would not build a rule on it.
Adding a reference point for repeatability versus intermediate precision. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.
I had read the opposite somewhere and cannot now find where, which tells me something.
Post #27 describes the usual case. This is about the unusual one.
One more thing on repeatability versus intermediate precision that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.