The rankings system is based on a pitcher’s Game Score, and although it weighs recent performances more heavily than past ones, it also considers the consistency and longevity of a pitcher over extended periods of time. A full explanation of the process can be found here at Bill James Online, although the basics of it are that for each game pitched, 3 percent of a pitcher’s total is taken away and 30 percent of his Game Score from that game is added to it. This results in approximately 63 percent of a pitcher’s total score being based on the past year, with the other 37 based on what he did before then.
Although Scherzer only recently passed Kershaw, the two have been close for a while now. Kershaw started the season with a score of 564.3, with Scherzer within striking distance at 550.6. Scherzer’s Game Score of 78 on Friday was enough to propel him to the top spot after weeks of catching up, taking advantage of Kershaw’s score of 59 against the Indians in his most recent start on Tuesday to finally make the move.
Naturally, there was some disagreement when Bill tweeted on Saturday about Scherzer’s ascent. For further discussion and an explanation behind the ranking, see Bill’s post here. Based on the response to that post, he also added another one today defending his choice of 3 percent decay in his system.