Measuring skin over twelve weeks, not twelve days

Change one product at a time and hold it for twelve weeks. Anything shorter reports noise back to you as a result.

Fix the protocol before the routine

Photograph under the same light, at the same time of day, at the same distance. Without a fixed protocol the comparison is meaningless.

Separate irritation from outcome

Record irritation on its own scale. A routine that works but is abandoned in week three has an effect size of zero.

Adherence is a result, not a personality trait. Design for it.