The author states that "better preoperative health tends to be associated with smaller, not larger, health gains" and refers to Fig 3, which shows a correlation of around r=-0.57 between preoperative health state and health gain. But this correlation is likely to be spurious.

Because 'health gain' was calculated as the difference between post- and preoperative health state (postoperative-preoperative score), Fig 3 effectively shows the correlation between a variable, x, and y-x.

r[x.y-x] is weighted by r[x.y], which is likely to be moderately to strongly positive, being the correlation between pre- and postoperative scores, and r[x.-x], which is by necessity -1.

Hence, any correlation between preoperative score and ‘health gain’ is nearly always negative, and significantly so if measurement error is sizable, as tends to be the case with PROMs data. This should not be interpreted as a differential effect of treatment, but as an artifact resulting from an index of health gain that is itself dependent on preoperative status.

The author is right to warn of statistical pitfalls when analysing PROMs data: indexing ‘health gain’ as the difference between pre- and postoperative scores should perhaps be added to the list.

Competing interests: None declared