When do stats become meaningful?

Pizza Cutter over at Most Valuable Network’s Statistically Speaking penned two articles this off-season regarding small sample sizes and when statistics start to become meanigful. Hat tip to Brock for Broglio for pointing this out.

What Pizza Cutter does is find out at which point the r-squared for different statistics reaches the .50 barrier, the point at which the majority of the variation of a stat is predictable.

The articles are long, but I definitely recommend taking the time to read them. Here is the article about hitters, and here is the article about pitchers.

Here is a list of the stats that have fantasy baseball implications, although by reading the articles you can get a better feeling for what this all means and just how stable these stats are at the given number of plate appearances/batters faced.

Hitters

Strikeout rate/Contact rate*: 150 PA
LD%: 150 PA
Walk rate: 200 PA
GB%: 200 PA
GB/FB: 200 PA
FB%: 250 PA
Home run rate: 300 PA
HR/FB: 300 PA
BABIP: Doesn’t reach a 0.50 r-squared at 650 or below.
Batting average: Doesn’t reach a 0.50 r-squared at 650 or below. Pizza Cutter guesses it would at around 1000 PA.
*Note: Pizza Cutter also lists a stat called “contact rate,” which stabilizes at 100 PA, but this is different than the contact rate that we use. I believe this refers to contact rate on a per-pitch basis as opposed to a per-at-bat basis.

Pitchers

K/PA: 150 BF
GB%: 150 BF
LD%: 150 BF
FB%: 200 BF
GB/FB: 200 BF
K/BB: 500 BF
IF FB%: 500 BF
BB/PA: 550 BF
BABIP: Doesn’t reach a 0.50 r-squared at 650 or below.
HR/FB: Doesn’t reach a 0.50 r-squared at 650 or below.

Concluding thoughts

I think this should help put early season stats into perspective. No batter has reached even 150 plate appearances yet, and no pitcher has faced 150 batters yet. So please relax, take a deep breath, and realize that even though April is 3/4 over, sample sizes are still too small to start making decisions based off of them.

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