[wenc]

> built Amazon’s recommendation algorithm, and it’s largely trash

What is trash for you may not actually be reflective of how good the algorithm is. My experience is that Amazon's algorithm actually works somewhat well.

Recommendation algorithms usually work well for people who are closer to average (i.e. their preferences being closer to the aggregate). People who deviate from the average tend to get poorer recommendations. It does even worse for segments that deviate so far from average where there are few sample points.

Recommendation algorithms optimize for aggregate conversion -- if it can get the most number of people to convert, it is good (for the merchant) even if it looks like your personal recommendations suck. It's all a matter of perspective on what "good" is.