But it seems the Quality ranking does not really work.

For example, search for the term "decimal" and sort by quality [1]. The first package listed is decimal [2]. This seems suspicious; there are only 77 downloads for this library in the last day, whereas other similar libraries have thousands of downloads. Now you might say that popularity is not necessarily a good measure of quality, but in the read-me for this package, I see this (under the header "Can I help":

_Of course you can, I suck at math, and this implementation is very naive. If you are a math Guru and you see something wrong or a way to simplify things you can send in a pull request._

So how exactly does np determine the quality of packages?

[1] https://www.npmjs.com/search?q=decimal&page=1&ranking=optimal [2] https://www.npmjs.com/package/decimal