Last week, I talked about DIPS Theory, its relevance to pitchers, and the fantasy implications that can give you an edge over your competition. If you recall, I digressed a couple of times to say that home runs are not completely within a pitcher’s control. I’d like to talk a little more about this today.

To measure a pitcher’s home run prevention, there are a couple of statistics we can use. The first is the method we use to typically measure strikeouts and walks, which is per nine innings, calculated as follows:


The second stat we can use is the Home Run per Flyball statistic. It is a simple ratio that is calculated as follows:


If we test the year-to-year trends of the two (using 2004-2007 data), we see that HR/9 has a correlation coefficient of 0.33 and an r2 of 0.11. For HR/FB, the correlation coefficient is 0.18 and the r2 is 0.03. These tests are statistically significant and show that pitchers have a little control over their home run rates, but that control is not great. Much of it is unexplained.

To help explain it, and to forecast future performance in this area, we turn to the HR/FB stat. Tests show that pitchers tend to regress to the mean, which is typically around 11-12%.

Pitchers have a little bit of control over their HR/FB rates, and home ballparks can explain a small portion of deviation, but pitchers who are significantly above or below this mark should be expected to regress.

As a quick example, let’s check out the pitchers in 2004 that had the lowest HR/FB rates and their corresponding HR/FB rates in 2005. To qualify, a pitcher needed at least 12 starts in both years.


As you see, they all got higher, and most of them put up HR/FB rates near 11% in 2005. Of course, just because a pitcher was heavily affected by luck in 2004 doesn’t mean that he can’t be heavily affected by luck in 2005. Hudson’s luck actually went the other way. But that’s why they call it luck. It happens, it is unpredictable, and the best we can do is assume that it will be neutral.

Here’s another thing to keep in mind when using HR/FB to make your own assessments. It is not uncommon for pitchers with extreme ground ball rates to have slightly higher HR/FB rates. Below is a breakdown of the league average HR/FB and the average among pitchers with at least 50 innings pitched and at least a 50% and 53% expected ground ball rate. The differences aren’t enormous, but you should be a little more lenient with these guys.


So which players’ ERAs figure to significantly change when their HR/FB rates regress? In the following tables, I’ll also include the pitcher’s ERA and xFIP to highlight the affect that these rates can have on an ERA.

I talked a little about xFIP last week; it measures a pitcher’s ERA using his peripheral stats while normalizing his home run rate. There is a stat that I like better than xFIP – LIPS ERA – which I’ll talk about at some point in the future (in three weeks, probably), but for now xFIP does the trick. First, let’s check out the HR/FB losers.


Some people will look at Chris Young’s HR/FB rate and quickly write it off as a function of Petco Park. There is no possible way, however, that a ballpark caused his HR/FB rate to be 7% lower than league average. It’s just not possible. There were other factors at play, and we shouldn’t expect him to put up anything lower than a 9% mark in 2008. Here are the winners.

Like I said last week with BABIP, don’t take these numbers at face value. Make sure that they are supported by solid skills. Felix Hernandez has excellent skills, and the HR/FB rate in 2007 serves to suppress his value going into 2008. He is a guy who should probably be targeted. Dontrelle Willis, on the other, has declining skills, and it is now being said he’ll be moving to the more hitter-friendly American League. Despite his higher-than-normal HR/FB, he figures to go higher in fantasy drafts than he deserves to.

Also, HR/FB is not the only luck component of ERA, so the affect of it won’t always be as apparent as it is in most of the cases above. In some cases, a pitcher’s xFIP and/or LIPS ERA will be higher than his actual ERA, even with an unlucky HR/FB rate. In these cases, there are likely other indicators (like BABIP, for example) that are acting even more powerfully. HR/FB does have a significant affect, though, and should be considered by all fantasy owners who take winning seriously.


That wraps up Stat Head for this week. If you have any questions about HR/FB or anything else in this vein, feel free to send me an e-mail. Next week I’ll probably talk about Left on Base Percentage (also known as Strand Rate), which is another critical indicator for pitcher ERAs.