He has run blinded home-use tests of the 10 top-selling shampoos and conditioners. Pantene turned out to make the best performing product.

Moreover, he found that expensive shampoos don’t work better than cheap ones, but expensive conditioners work better than really inexpensive ones, like V05 or Suave, he said. That’s because the pricey conditioners have more types of nourishing ingredients, which all work differently, and together are more effective.

Still, he said, “super-expensive ones aren’t better than Pantene.”

But this is where things get knotty. A major driver of whether a product works — or really, whether a customer thinks it does — is fragrance, which of course has no visible effect (unless you’re allergic to it). In its early days, Function of Beauty had to spend more time tweaking scents than algorithms.

“Man, people care a lot more about fragrance than I ever thought they would,” said Mr. Dossa, who, when he started the company, had a shaved head, but now has a man bun so he can test products. “Customers won’t properly assess your product if they’re not happy with the fragrance.”

Prose does not offer customers who want a moisturizing shampoo the option of choosing a citrus fragrance. Mr. Plas learned from his time at L’Oréal that customers will insist the product doesn’t work because they associate that smell with grease removal. “Then we give them the exact same formula with a different scent, and they think it’s great,” he said, shaking his head.

Another thing customers can’t (yet) choose easily: to ban a particular ingredient because of allergies and sensitivities. (Function of Beauty is vegan and gluten free; Prose allows you to choose those options. Both allow you to avoid fragrance, but you have to contact customer service to ban a specific ingredient.)




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