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Rule Changes: The New MiLB rules and possible implications for MLB play

MLB announced that MiLB would be the test lab for several rule changes that may one day make it to Majors. What are the rule amendments? How could they impact the game generally and the Toronto Blue Jays specifically if MLB implemented the new rules for the 2021 MLB season?

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On March 11, MLB publicized some rule changes for the 2021 MiLB season. MLB intends to determine if the changes will create more action and improve the pace of play. Another objective is to increase player safety.


The rule changes are as follows:

  • Larger bases
  • Defensive positioning (modified shift restrictions)
  • Pickoff attempts
  • Pitch clock
  • Robot umpires


For a detailed description of which leagues will adopt the various experimental rules, please refer to this article.


Larger bases

The new rule calls for an increase in a base’s size from 15 inches square to 18 inches square. There will also be a change to the surface that will make the base less slippery in wet conditions, improving player safety. To determine the impact of a larger bag upon base runners, we have first to review the bases’ location on a diamond. This diagram illustrates the placement of bases relative to home plate.


So what does the larger bag mean for on-field play? Well, the distance between the edge of first base (the one closest to home) and home will decrease by three inches with the new bag compared to the 15-inch version. Similarly, third base will be three inches closer to home than with the current base dimensions. The distance between first and second base (measuring from the two edges closest to each other) will be four and a half inches less than they are with a 15-inch base.  The same is true of the gap between second and third.


Running from home to first

According to Statcast, the average time for a batter to run from home to first base was 4.48 seconds in 2020. With first base three inches closer to home, the average runner would take 4.47 seconds, a savings of 1/100th of a second. That is not much, but it may result in some more infield hits throughout a season.


Running from third to home

During the 2020 season, the average base runner covered 26.8 feet in one second. I applied this number to the tag-up play from third to home, which produces an average time to run from third of 3.32 seconds with a 15-inch base. The larger base will reduce that time to 3.31 seconds. Hence, a few more sacrifice flies may occur if an 18-inch bag is utilized compared to the 15-inch model.


I will examine the impact of the larger base upon base stealing in the Pickoff Attempts section.


Defensive Positioning

The defensive-positioning rule change is not a complete ban of the shift. Teams would be permitted to have more than two infielders on either side of second base. The rule amendment would require teams to have all infielders positioned on the dirt portion of the field. For example, an infielder would be prohibited from standing on the grass in right field.


In the article, The Infield Shift: The Toronto Blue Jays and the Case Against, I addressed the impact of a full shift ban. I concluded that a full ban would have little impact on offence. The estimate was that the number of hits would increase by 1.6% and the number of runs by 2.0% (assuming all hits were singles). When I applied the ratio of singles, doubles, and triples from the standard infield alignment to the estimated increase in hits, runs would rise by 2.3% instead of 2.0%.


Under the modified shift ban rule contemplated for MiLB, the number of hits and runs should be less than if a total ban was in place. Therefore, not very impactful.


Pickoff Attempts

Stealing bases

The modification of the pickoff rules should result in more stolen-base attempts. However, we have first to address Sabermetrics and its effect upon on-field play. No, analytics did not kill the stolen base, but its use as an offensive tool has declined in recent seasons. In 2010, there were 4,088 stolen-base attempts, 3,569 in 2015, and 3,112 in 2019. Of note, the stolen-base success rate has hovered around the 72.5% figure during the 2010-2019 period.


The Run Expectancy Matrix is likely the primary reason why teams attempt fewer base steals and sacrifice bunts than they did 20 years ago.  Table 1 is the current version of the matrix. For a detailed review of the matrix, including how it works and the implied success rate inherent in deciding to attempt a steal, refer to Appendix A.


According to the current publicly-available Run Expectancy Matrix, the threshold success rate for stealing second base is 71.5%. Generally speaking, if the Manager thinks that the probability of success is greater than 71.5%, they should give the runner the go sign. Let’s now determine if some of the rule changes could create more stolen base attempts.


Speed Estimates

How fast does a runner have to travel from their lead off first base to second base for a successful steal attempt? To answer that question, I examined the combined time for a pitch to reach home and the catcher’s throw to arrive at second base. John Dewan wrote an article on the subject. He demonstrated that if the combined time was between 3.40 and 3.55 seconds, the success rate on stolen base attempts was 73.9%. This success rate is close to the 71.5% threshold rate noted earlier. Hence, I used 3.40 seconds for my analysis.


Another factor to consider is the length of the lead that a runner takes. Pro Ball Insider stated that MLB players take a lead in the 9 to 12-foot range. Furthermore, the players with the highest Sprint Speed metrics cover 80 feet in approximately 3.40 seconds. For purposes of the analysis to follow, I assumed that a player would take a nine-foot lead. This lead places the runner about 80 feet from second base (79.125 feet = 90 feet less the 9-foot lead less the 1.875 feet of the bases [from the foul line to the edge of first base plus half of the width of second base]).


Base stealing and the larger base

I estimated that, with a 15-inch base and a 9-foot lead, the best runners cover almost 80 feet between first and second in 3.40 seconds. If the base is 18-inches square, then the time to cover the distance between first and second on a steal attempt is 3.38 seconds. Hence, the increase in the base’s size will likely have a negligible impact on the success rate.


Step-Off Rule

There is good news. Under the step-off rule amendment, all pitchers must step off the rubber before throwing to a base. The Atlantic League tested this rule in 2019. The result was that runners were more aggressive with their leads and more successful in stealing bases. Let’s do some math!


Assume a player, who takes a nine-foot lead off first, will cover the almost 80 feet in 3.40 seconds. This time translates into 23.5 feet per second. Let’s assume that the step-off rule gives the base runner the confidence to take a 12-foot lead. Therefore, the same runner travelling 23.5 feet/second will arrive at the second base’s edge in 3.27 seconds. If we add the 18-inch base into the mix, the time it takes to reach second would be 3.25 seconds. Together, the larger bag and the step-off rule could reduce the time to reach second by 15/100ths of a second. That is impactful. Based upon the Dewan article, the success rate could optimistically increase by as much a five percentage points (73.9% less 68.7%).


Accordingly, reducing the estimated time to reach second on a steal attempt should increase the number of steal attempts. The matrix can still be a friend to lovers of base stealing!


Another rule change is that pitchers will be limited to two “step offs” or pickoff attempts per plate appearance. Once the total of two is reached, or possibly after the first move, the base runner can be very aggressive with their lead from a base. Therefore, these new rules should contribute to an increase in the number of stolen base attempts.


Pitch Clock

The Double-A and Triple-A levels have used a 20-second pitch clock since 2015. Initially, the average length of a game decreased by 12 minutes. However, the average time has crept up in recent seasons. The 2021 rule change will see the introduction of a 15-second pitch clock.


Robot umpires

Under the rule amendment, MiLB will use an automated balls-and-strikes system. However, a human umpire, positioned behind home plate, will receive an audio signal informing them whether the pitch taken was a ball or a strike. The ump will continue to make the call whether or not the batter swung at the pitch.


Newsflash! MLB umpires are not perfect at calling balls and strikes. In April 2019, Boston University published MLB Umpires Missed 34,294 Ball-Strike Calls in 2018. Bring on Robo-umps? The authors studied the strike and ball calls made by MLB umps during the 2018-2018 period. The highlights of their findings are as follows:

  • Umpires have a two-strike bias. With a two-strike count, umpires were twice as likely to call an actual ball a strike than when the strike-count was lower. In 2018, 21.5% of called third strikes were balls.
  • During the 2018 season, the error rates by strike-zone sections were as follows: top left – 26.78%. top middle – 13.55%; top right – 26.99%; bottom left – 14.34%; bottom middle – 10.50%; and bottom – right – 18.25%.
  • The average overall error rate for MLB umpires was 12.78% during the 2008-2018 period; it was 9.21% in 2018.


At this date, it is not possible to assess with confidence what the impact of the correct ball and strike calls will have upon the game. One thing for sure is that if MLB were to adopt robot umps, the pitch-framing metric would disappear. Why? Because catchers will not be able to trick umpires into calling a ball a strike. One question will be how batters will adjust to more called strikes in the strike zone’s upper part. In 2019, MLB’s batting average was 0.254. This diagram shows that batters in 2019 produced a 0.219 batting average when hitting a pitch in the top-left portion of the strike zone (catcher’s view).


The Blue Jays

Let’s bring the MiLB rule changes closer to home. Assuming the proposed MiLB rules were implemented this season for MLB play, what could the impact be upon the 2021 Blue Jays?


The 18-inch base may marginally benefit players on infield batted balls and on tag-up plays. Accordingly, Blue Jays in the 20th percentile in Sprint Speed would most likely benefit from the larger base. Those players are Marcus Semien and Teoscar Hernandez.


The step-off rule may negatively affect Hyun Jin Ryu because he is very good at holding runners on first. I have used the rate of stolen base attempts as a proxy for how well pitchers hold runners. The tactics used to hold runners include not stepping off the rubber before throwing to a base. In Ryu’s career, with the bases empty except for a runner on first, base runners attempted to steal second base in just 7% of the situations. The comparable figures for Tanner Roark and Steven Matz are 28% and 49%, respectively. If MLB invoked the step-rule, Ryu, in particular, may see an increase in steal attempts.


On the other hand, Toronto base runners may become more aggressive in the base-stealing department. Two candidates who could profit from the step-off rule are Cavan Biggio and Hernandez due to their career stolen base success rate. Biggio is 100% in 20 career attempts, and Hernandez is 75% in 16 attempts. Biggio can’t do better than perfection, but he and Hernandez may take advantage of the new rule and try more thefts of bases.


Danny Jansen may not enjoy some of the proposed rule changes. For the 2019-2020 period, his pitch framing prowess produced an above-average Runs Extra Strikes metric (3). Robot umps will eliminate any value generated from pitch framing. Also, suppose the step-off rule increases the number of steal attempts. More base-stealing tries will further expose Jansen’s deficiencies in throwing to bases to nab runners. In 2019 (2020 data not available), Jansen ranked 23rd in Pop Time (catchers with a minimum of 20 attempts).


The last word

MiLB will implement some rule changes for the 2021 season. The amendments range from reducing the size of the bases, a partial ban of the infield shift, modifications to the current pickoff moves, and robot umpires. The most impactful changes will likely be the rules concerning pickoff rules, which should increase the number of base stealing attempts, and robot umpires. If successful, MLB may introduce the new rules for play in the Majors in the future. The new rules may improve the pace of play and create more action on the base paths.




*Featured Image Courtesy Of DaveMe Images. Prints Available For Purchase.




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Bob Ritchie

Bob was a St. Louis Cardinals fan until the Blue Jays arrived on the baseball scene, although he still has a soft spot for the Cards. Similar to straddling the Greenwich Meridian, as depicted in the avatar, Bob applies sabermetrics when applicable, but his heart tells him that Lou Brock belongs in the Hall of Fame.