Monday, July 16, 2007

Bonds' steroid cycles -- a Tom Tango study

A few days ago, I posted a review of Derek Zumsteg's study of Barry Bonds' hitting patterns, which, Zumsteg argued, showed evidence that Bonds was on a steroid cycle.

It turns out that Tom Tango studied the issue in much more detail over three months ago, which I missed. (You'll have to read the comments
here to find his studies.)

The original idea was this: if Bonds was on steroids for three weeks, and then off them for one week, we should find that his performance similarly cycles over 28 days – three weeks better hitting, one week worse hitting. Comparing the years Bonds was allegedly on PEDs to the years he was off, Zumsteg found the cycles were larger in the steroid years.

Tango built on that by looking at *all* players over several years, and examining their own cycles. If Bonds was juicing and most other players were not, his cycles should be much more pronounced than those of the clean players.

It turns out that if you compare cycles by looking at the difference in wOBA (Tango's stat) after subtracting "off cycle" from "on cycle", Bonds is very near the least consistent, suggesting PED use. But if you look at the *ratio* of the two stats (by dividing rather than subtracting), his cycles become much more ordinary. As Guy points out in comment 39 of Tango's post, he's only 1 standard deviation above average.

Guy is probably right, that ratio is a better indicator than difference. It's a lot easier to gain X points of OPS by hitting a few more home runs than to gain the same number by hitting singles.

And so I think Bonds' showing in Tango's excellent study doesn't really constitute evidence of juicing. And while I might be biased in favor of my own work, I think the simulation results in my
previous post are good evidence that Bonds' 2002 cycles are almost exactly what you would expect by chance.

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At Tuesday, July 17, 2007 11:20:00 AM, Anonymous Anonymous said...

I agree. Your analysis in the previous post is right on. Another trap that Zumsteg and to a lesser extent Tango seem to be running into is that variability of max vs. min weekly cycles is dependent on the ratio of HR a player hits in general.

For example, suppose Ichiro and Bonds have the same wOBA - almost certainly Bonds will show greater cyclic variation because a few HR in bunches (or dearth of HR in bunches) goes a lot further in varying the wOBA than a few singles in bunches.

Does that make sense?

At Tuesday, July 17, 2007 11:25:00 AM, Blogger Phil Birnbaum said...

Yup, that makes a lot of sense. I wonder if it can even account for the fact that Bonds had a higher-than-expected *ratio* of on/off?


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