What Do Minor League Walk and Strikeout Rates Tell Us About Prospects? Rookie League and Low A Edition

By Chris St. John


Introduction
A few months ago, I created a database that includes all of the prospect rankings from Baseball America, Keith Law and Kevin Goldstein. I didn't have a direct purpose for it, I'm just the type of person who likes to accumulate as much data as possible. So it sat around on my computer until I found a good use for it. Well, thanks to fantasy baseball and Starling Marte's horrible 3.8% walk rate in AA last season, I have. Marte is a possible target in my dynasty minor league draft and I wanted to see what other prospects had poor walk rates and how successful they were in their careers.

Method
I decided to only look at hitting prospects who appeared on a Baseball America list from 1990 to 2006, a total of 480 players. This provides an ample amount of time for us to know how the prospect fared in his career. Then, I took minor league plate appearance, walk and strikeout data from 1987 to 2011 from Baseball Prospectus. The Rookie league data may not be complete, but should be extremely close. Then I added Fangraphs WAR and other assorted Major League offensive metrics for each prospect.

I removed all minor league data after a player accumulated 100 Major League PAs, removing rehab and non-prospect years. Then I created level average metrics by year: BB/lvlBB*100 and SO/lvlSO*100. This doesn't account for the various leagues (International versus Pacific in AAA) but it's better than nothing. I calculated a weighted average for players who spent multiple years in the same level.

Next, I wanted to account for small sample sizes, so I only looked at players who accumulated more than 150 total plate appearances per level. Then I calculated a variance for each player from the average for qualified players at each level for both walk and strikeout rate: (BB/PA+ - average BB/PA+ for qualified players)/Standard deviation BB/PA+. Finally, I split players into categories of Very Low, Low, Average, High and Very High, based on this variance.


I also split players into three levels of Major League success:
Productive Hitter (100 total): More than 1500 ML PAs and Battting Runs per PA greater than 0.166
Average Hitter (101 total): More than 1500 ML PAs and Batting Runs per PA between 0.166 and -0.0023
Bust (279 total): Fewer than 1500 ML PAs or Batting Runs per PA less than -0.0023

The Batting Runs qualifications are based on the mean and standard deviation of career numbers for prospects. The line between Productive/Average is at Ryan Ludwick/Jeromy Burnitz and the line between Average/Bust is at Tadahito Iguchi/Rich Becker. This distinction leads to three very important statistics: 21%, 21% and 58%. That's the percentage of productive, average and busted hitting prospects, respectively. Of course, this ignores defense and position, which may be where a player's value lies.


Rookie Leagues
Only 94 of the 480 prospects in this dataset accumulated at least 150 plate appearances in rookie ball. However, the Rookie League data may be missing some years and leagues, so I don't feel comfortable with calling this an exhaustive look for this level.

Walks

With such a small amount of prospects actually going to rookie ball, there are not many conclusions to draw here. The number of prospects with an average walk rate is by far the largest sample and is also the closest to the overall productive/average/bust percentage values. Of the nine prospects with the highest walk rates in rookie ball, two were productive (Prince Fielder and Jack Cust) and seven were busts (Rich Becker, Al Shirley, Mitch Meluskey, Roger Cedeno, Ricky Gutierrez, Michael Restovich and Pedro Castellano).

Of the 15 prospects with low walk rates in Rookie ball, five of them (Josh Hamilton, Carlos Lee, Andruw Jones, Raul Mondesi and Javy Lopez) ended up being productive hitters. The 33% success rate is much higher than anticipated, but again, the sample size is very small here.

Strikeouts

Of the 14 productive hitting prospects that played in Rookie ball, none of them had a low strikeout rate and all but two were average. The most productive hitter with the highest strikeout rate in Rookie ball: Russell Branyan, who struck out almost 1.7 times the level average in 201 PAs.

Strikeout to Walk Ratio
Finally, this plot shows the relationship between Rookie League K/BB and Major League Batting Runs per Plate Appearance:

Click to Enlarge

Low A Leagues
Only 111 of the 480 prospects accumulated at least 150 PAs in Low A. 14% were productive, 22% were average and 64% were busts.

Walks

No prospects with a low walk rate in Low A were productive major league hitters, while three of the 10 with high walk rates were (Ryan Howard, Ben Grieve and Jeremy Giambi).

Strikeouts

16 of the 21 prospects with high strikeout rates in Low A were busts. Only three were productive (Carlos Delgado, Ryan Howard and Tim Salmon).


Strikeout to Walk Ratio
Finally, this plot shows the relationship between Low A K/BB and Major League Batting Runs per Plate Appearance:

Click to Enlarge

There is a much more visible trend here than in Rookie ball, but the r^2 value is still extremely low.


Conclusion
While there are a few interesting data points, there aren't any good overall trends here with such small sample sizes. The total percentage of productive hitting prospects that accumulated at least 150 plate appearances in either Rookie or Low A levels is lower than the percentage for all prospects (15% vs. 21%). I'll continue the series with Class A, where the sample size is much larger. If you would like to know anything specific about the data, you can contact me in the comments section below or on Twitter @stealofhome.