All 1100+ songs that have ever hit #1 on the Billboard Hot 100 since its inception in 1958 are included. I chose that list because I believe it's a fairly objective way to compare pop music across eras instead of using subjective curated lists.
You can vote as many times as you want. Given the sheer number of songs, you'd go quite a while without seeing a repeat matchup.
Songs are randomly picked without regard to era or artist, weighted by the number of weeks they were at #1. This means that a song from 1960 is as likely to come up as one from 2020. In addition, you will not see the same song twice in short succession.
Ratings are calulated using a maximum likelihood estimation. Essentially, a score of 5 is "average", and the higher the better. In theory it's possible to have a negative rating or one over 10, but they would be hard to achieve (and values of 0 and 10 don't actually have any meaning anyway). In addition, each song is given one "win" and one "loss" against John Cage's 4'33" to prevent small sample size issues.
Youtube embeds for this many songs can be hard to maintain. Please let me know and I will try to fix it ASAP.
My general rule of thumb is to find official music videos of songs that hit #1 after the launch of MTV. There are some older songs with videos, but it can be hard to tell which are contemporaneous, which are promotional videos done years later for Youtube, and which are fan videos.
Yep.