Where the "11 times stronger than retinol" claim comes from

This 1990 biochemistry paper measured how quickly cultured human skin cells convert retinol and retinaldehyde into retinoic acid, the active form of vitamin A. In differentiating cells, retinoic acid formed at 4.49 units per hour from retinol and 51.6 units per hour from retinaldehyde. Divide the second by the first and you get 11.5 — and that is where the widely repeated claim that retinal is eleven times stronger than retinol comes from. The paper itself never uses the word eleven. It is worth understanding exactly what was and was not measured. This is a conversion rate in cell extracts, with cofactors supplied, which tells you that retinaldehyde has already cleared the slow first step that retinol has to get through. It says nothing about how much of either ingredient survives in the bottle, penetrates skin, or improves how skin looks. The same paper also notes that retinaldehyde gets converted backwards into retinol, which the marketing never mentions.