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Thread: How Big of a Sample is Big Enough?

  1. #1
    Stat Wanker Hodiernus RedsManRick's Avatar
    Join Date
    Dec 2004
    Guelph, ON

    How Big of a Sample is Big Enough?

    BP just released an updated version of its study about when certain metrics become more signal than noise.

    I HIGHLY recommend you read the full article for a comprehensive discussion, but here's the latest version, sorted by denominator and sample size.

    Statistic	Definition		Stabilized at		Notes
    GB*rate		GB/BIP			80 BIP			Min 1000 BIP, Retrosheet classifications used
    FB*rate		(FB+PU)/BIP		80 BIP			Min 1000 BIP including*HR
    HR*per*FB	HR/FB			50 FBs	(~125 BIP)	Min 500*FB
    LD rate		LD/BIP			600 BIP			Min 1000 BIP including*HR, Estimate*
    BABIP		Hits/BIP		820 BIP			Min 1000 BIP,*HR*not included
    Strikeout rate	K/PA			60*PA	
    Walk rate	BB/PA			120*PA			IBB's not included
    ISO		(2B+2*3B+3*HR)/AB	160*AB			Min 2000 ABs, Cronbach's alpha used	
    HR*rate		HR/PA			170*PA	
    HBP*rate	HBP/PA			240*PA	
    Single rate	1B/PA			290*PA	
    SLG		(1B+2*2B+3*3B+4*HR)/AB	320*AB (~350 PA) 	Min 2000 ABs, Cronbach's alpha used, Estimate*
    OBP		(H+HBP+BB)/PA		460*PA	
    AVG		H/AB			910*AB (~1000 PA)	Min 2000 ABs
    XBH rate	(2B+3B)/PA		1610*PA			Estimate*
    For the same of argument, let's just that about 2/3 of plate appearances, on average, result in a ball in play. So you can multiply the BIP figures by 1.5 to get a rough comparison to PA.

    My big takeaway: There's a reason the three true outcomes are given the most attention. They stabilize the most quickly, suggesting some combination of two things:

    1) They are inherently more a function of skill than the other metrics
    2) The skills they are indicators of have less natural fluctuation than the skills that the others do

    Not surprisingly, the more other people (defense) and things (park effects) affect the outcomes, the bigger the sample needs to be to tell you about the skill the player himself possesses.

    One easy way to interpret this is thus: If we don't have a big enough sample yet (per this table), then the players career stats (or a projection based on them) are a better predictor of how he's likely to perform moving forward than how he's performed in this sample.
    Last edited by RedsManRick; 07-16-2012 at 03:09 PM.
    Games are won on run differential -- scoring more than your opponent. Runs are runs, scored or prevented they all count the same. Worry about scoring more and allowing fewer, not which positions contribute to which side of the equation or how "consistent" you are at your current level of performance.

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  3. #2
    High five! nate's Avatar
    Join Date
    Sep 2005
    Irvine, CA

    Re: How Big of a Sample is Big Enough?

    Excellent post, Rick.
    "Bring on Rod Stupid!"

  4. #3
    All work and no play..... Vottomatic's Avatar
    Join Date
    Oct 2009

    Re: How Big of a Sample is Big Enough?

    I thought this was gonna be a thread about Stubbs.
    "I can't take this homerism anymore." - 10xWSChamps, August 11, 2010. A Cardinals fan having a problem with all the homerism on Redszone. Classic.

    "Man do I miss the days where were didn't need a calculator and an encyclopedia of baseball metrics to enjoy a baseball game ... - MikeS21" - 8/2/12 game thread

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