To make that happen, Prose uses a method preferred by many personalization start-ups, including Care/of, which doles out nutritional supplements, and Curology, which mixes custom creams: an internet survey. I love talking about myself, so taking a lengthy quiz that would assure me of my hair’s snowflake status wasn’t exactly a tough sell. Plas says the brand now gets more than a million survey submissions a month.

Even so, asking people to describe their hair—or their skin, or their eating habits—has limitations. “It’s very important not only to ask the questions, but how you ask the questions,” Plas cautions. “We’re realizing that some people don’t really know how to answer.”

According to Holly Maguire, the creative director at the cosmetic-development company Freelance Formulations, people have a difficult time situating themselves on a continuum when they don’t have much experience with other people’s skin or hair. “I think everyone’s perception of themselves and their skin and hair and how they look varies,” she says. “What constitutes oily hair? How do you tell if you do actually have oily hair, and what level is it?”

In reality, my paradoxical hair isn’t as special as I’d like to believe. Lots of people bleach and heat-style, leave their hair in a ponytail holder for a day or two, or go a little overboard with the dry shampoo. I simply believe my hair is special because it’s my problem. While taking my own survey on Prose’s website, my potential to be an unreliable hair narrator (hairrator?) became clear. Prose asks about texture and moisture, but also about your environment, your activity level, and how much time you spend outside. I live in New York City and walk a lot, but I don’t exercise outside—unless exercise includes walking a lot? Prose also wants to know how you eat, which means I had to divine the practical difference between a diet that is “balanced” and one that is “unrestricted.”

To get the most out of personalization via algorithm, not only do you have to know the ins and outs of your own needs, but you have to think about what a brand might be trying to learn by asking a particular question, and how that information might impact your results. It’s the e-commerce version of swiping your hand under an automatic faucet, trying to determine what kind of motion the machine wants in order to get your hands wet. These services can only go as far as the partnership between user and technology allows, which means that if you do a half-assed job answering the survey, you could end up with a multivitamin or skin-care routine no better than what you would have bought at a store.

That doesn’t mean personalization programs can’t have upsides for both consumers and brands. Maguire sees advantages to using personalized products, even though her company creates things for the mass market. “When you’re talking about skin sensitivities or allergies, I definitely think that a custom product could be needed,” she says. Personalized beauty-product brands frequently let customers opt out of fragrances or ingredients derived from animals, which gives shoppers with sensitivities or ethical preferences more options than they would have otherwise. In addition to the lengthy up-front explanation of which ingredients are in each product, Prose follows up with buyers after three weeks to tweak their formulas as needed.