Football League Rankings uses data from the awesome
Global Club Soccer Rankings by
FiveThirtyEight!
How does it work?
Pretty simple! It takes the rankings of all clubs from FiveThirtyEight, groups them into what league they belong too, and calculates the average SPI (Soccer Power Index) and the standard deviation of the values!
For more information about SPI ratings and how the ratings work in general - check out FiveThirtyEight's overview
here.