Tyler Breland: DataScience solution

So, this pretty well works as long as `cohort_visits` is itself immutable. In this case, it looks like you're not storing it, so that works. Just keep in mind that you've got all the attributes as `attr_accessor` (implicitly readable) and if you had an API that supported adding new visits, your attributes would fall out of sync.

It's typical to just use methods to compute this stuff on the fly, or to memoize it if it's too expensive to compute.