http://a.co/a8yEzzt A Manga Guide to Statistics. No I'm not being snarky, it's a great read and I highly recommend it. Sampling, why it works and the philosophy behind statistics are all a bit unintuitive and so a light read is perfect for helping you develop that intuition.
I have a degree in mathematics, I'm fine on the statistics front. It's very likely to NOT be a random sample - you would only be testing the packages which have had effort put in to make them reproducible, for reproducibility. I would expect the non-sampled packages to be 0% reproducible, since the reason they were not tested is that nobody is trying to make them reproducible (yet).
