math

New Feature: Aging Curves (2016)

A question that tends to pop up around this time of year: “When does fantasy baseball season start?” Of course, we all know that fantasy baseball season never ends, especially for those of us in keeper and dynasty leagues. To wit, Brad Johnson’s “Keeper Questions” thread posted just yesterday at FanGraphs is now sitting at 350 comments and growing. As we all collectively count the days ‘til spring training and opening day, one of the most oft-discussed and most subjectively-answered topics is “Who do I keep?” Fantasy baseball players intuitively understand the idea of aging, at least qualitatively. Older players are less valuable, given that their performance is more likely to decrease due to both injury and ineffectiveness. But how much is age worth, really?

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Breaking Down the BIGz Equation for z-Scores

z-Score: a statistical measurement of a score’s relationship to the mean in a group of scores. A z-score of 0 means the score is the same as the mean. A z-score can also be positive or negative, indicating whether it is above or below the mean and by how many standard deviations. 

After throwing around the terms z-score and BIGz somewhat haphazardly on this site, I figured the time has come to explain where it all comes from. For the math-inclined individuals among you (and you play fantasy baseball, so I’m not sure why I have to qualify that), this will hopefully satiate your need to understand where all the numbers in the Big Board come from. This is also going to theoretically empower you to create your own Big Board, but hey, save yourself some (read as: A LOT of) time and buy the one hosted on these very interweb-pages. More generally, this breaks down one of the more popular methods for creating player rankings and dollar values, sometimes referred to as ‘FVARz’ for Fantasy Value Above Replacement z-scores as popularized by Zach Sanders of Fangraphs.

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