sport performance.

Measuring the extent of the placebo effect is difficult due to confounding factors. For example, a patient may feel better after taking a placebo due to regression to the mean (i.e. a natural recovery or change in symptoms), but this can be ruled out by comparing the placebo group with a no treatment group (as all the placebo research does). It is harder still to tell the difference between the placebo effect and the effects of response bias, observer bias