It's also the structure that a formal class affords that makes it more effective. This is part of the appeal of book clubs (yes, part of it is, for some people,

It's also the structure that a formal class affords that makes it more effective. This is part of the appeal of book clubs (yes, part of it is, for some people, the opportunity to hang out with people and drink wine and eat cheese): some externally imposed structure makes it easier to commit to learning something. Paying for an opportunity also tends to make people more likely to honor their commitment.