Second pass at: Robustness: the parameters worth deliberately varying

The failure mode on Robustness is boring rather than dramatic. It is almost always the step everyone assumes was done correctly because it is too simple to get wrong.

A note on scope: what I am saying about Robustness applies to the case in the first post and I would not extend it further without checking.

That is clearer than the version I had in my head. Thank you.

An observation about Robustness that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.

I would call the community position on Robustness likely rather than established, and I would be comfortable defending that hedge.

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.

Practical note on Robustness: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.

The arithmetic in post #51 is right; the assumption feeding it is the part to check.

The distinction between a qualified instrument and a validated method is worth keeping. Both are needed and they fail in different ways.

Reporting the observation and leaving the explanation open deliberately.

Coming back to post #62, because the follow-up matters more than the original answer.

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.