The 2018 season was supposed to be Luis Robert's coming-out party. Instead, he was limited to 50 games across three Minor League levels after suffering two separate sprains to his left thumb. But there's one department in which the 21-year-old outfielder, who signed with the White Sox for $26 million out of Cuba in May 2017, stood out in his short time on the field -- speed.
Speed is great. It gets an athlete in the door. In almost every sport, a player who can get from Point A to Point B quickly has a leg up on the competition. But pure speed is one thing. The application of that skill is another.

In baseball, a few questions arise when it comes to utilizing speed: How often does a player try to steal a base? How effective is he at doing so? How often does he stretch a double into a triple? How often does he come around to score instead of being stranded on base? Instead of just using a stopwatch to measure home-to-first times, there are other ways to think about speed and how it affects play on the diamond.
Welcome to Speed Score.
Speed Score (or Spd) was initially designed by Bill James to measure a player's effective use of speed and baserunning ability on a scale of 0-10. Since he first came up with the stat in the 1980s, there have been a few different versions -- it's not like batting average, where the equation is constant -- but for the purposes of this Toolshed, we'll use the four-component model as explained by David Appelman. To paraphrase:
--Stolen-base percentage: How good is a player at stealing a base once he decides to go?
--Frequency of attempts: It's one thing to be perfect in stolen bases, but if perfect means 1-for-1, that doesn't tell us much. Speedy players are more likely to attempt to steal more often.
--Percentage of triples: Players with speed can turn singles into doubles, doubles into triples, triples into inside-the-park homers. Triples are most closely associated with speed and more frequent than inside-the-parkers, so this measures how often a player turns balls in play into triples. (This component typically results in a number between 0 and .03, but is put on a 0-10 scale.)
--Runs-scored percentage: By the same token, speedy players are more likely to wheel around the bases and score than slower runners, who could be thrown out or stopped prematurely and stranded at third.
Take the mean of those four components, and the final number is considered Speed Score. Here are the top five Minor League Speed Scores posted this season by players ranked among MLB.com's top 100 prospects: 

Spd leaders for top-100 Prospects

To put Robert's 8.7 Speed Score into context, the average Speed Score among top-100 prospects was 5.3, so he's well above that mark. Going even deeper, Victor Robles, whose 75-grade run tool is tops among this elite group, was 15th at 6.4. Myles Straw led the Minors with 70 stolen bases, has an 70-grade run tool and still fell short of Robert with an 8.3 Spd.
Using the average Speed Score and its standard deviation -- as the Toolshed on ISO did -- Robert actually performed like a 75-grade runner on the basepaths, a full grade above the 65-grade run tool he's been assigned from MLB.com. This is not to say that if Robert lined up and ran 40 meters next to Robles and Straw, he'd definitely beat them. But this does say that he was more effective at using his speed on the basepaths in 2018 than those two or any top-100 prospect.

How is that so? For one, Robert was elite at coming home. The 21-year-old reached base 69 times over 50 games between the Arizona League, Class A Kannapolis and Class A Advanced Winston Salem. He came around to score 32 times, despite not going deep himself. He and Fernando Tatis Jr. were the only prospects here who received perfect 10s in the runs scored category. While one could contend that a player scoring runs is dependent on having good hitters behind him, Spd is willing to give the runner some credit for using his own feet to scamper home, and given Robert's scouting profile, it's not a huge surprise that he could have manufactured some of those runs himself.
As for steals -- the stat mostly tied to speed by traditional stat-heads -- Robert may have gamed the system a bit. Three of his 15 thefts on the season came over a five-game rehab stint in the AZL, where he played from July 31-Aug. 5 upon returning from his second thumb injury. That's not to say Robert didn't earn those stolen bases, but stealing on Rookie-level pitchers and catchers may have been an easier way of adding to his tally. Excluding those, Robert's 12-for-16 success rate on the basepaths between his two full-season affiliates was good but doesn't necessarily jump out. Indeed, take out the AZL stint entirely and Robert's Speed Score at his two full-season affiliates drops to a still-respectable 7.5, in line with his 65-grade run tool.
Either way, Robert may not have been able to show off much of his hit tool (.269 average) or power (.360 slugging percentage) this season due to his thumb injuries, but his speed remained in full view. He'll make up for lost time in the Arizona Fall League, and if he continues to blaze the basepaths there, there will be many reasons for optimism entering his third Minor League season in 2019.
A couple other quick notes on Speed Score:
--By using the averages and standard deviations of both Speed Scores and run tool grades, Toolshed was able to devise, again like ISO, the expected Speed Scores for each of the top-100 prospects. By those measures, these were the biggest overperformers:

Biggest Speed Score overperformers among top 100 Prospects (min. 100 at-bats)

It's notable that three top picks from the 2018 Draft figure here. Although, like Robert, all three had shortened seasons and may have had their Speed Scores boosted by the fact that they played for Rookie-level or Class A Short Season clubs. Bart performed like a 55-grade runner but earned his biggest scoring component for triples after picking up three in 51 games. Same for Bohm, who tripled twice in 40 games. Both were drafted within the top three picks for the offensive potential at the plate, and neither the Giants nor Phillies will expect their newest top-100 prospects to burn rubber during their first full seasons in 2019.
Kelenic, however, should make an interesting case to follow next spring. The sixth overall pick in June had six triples and 15 steals in 56 games between the Gulf Coast League and Rookie-level Kingsport, and though he's considered a 55 runner, his Speed Score was closer to that of a 70-grade burner. If he stays aggressive and efficient with full-season clubs, he could add even more helium to his already rising profile.
--Tatis figures onto the overperformers list for a second straight season after leading it in 2017. The game's No. 2 overall prospect scored 77 times in 88 games for Double-A San Antonio and added four triples and 16 steals in that span. That's just another reason to be excited for his toolsy future and sad that the Minors didn't get to see more of his potential in 2018 after a fractured thumb cut his season short in July.
--At the other end of the spectrum, the top-100 prospect with the lowest Speed Score in 2018 was Dodgers catcher Keibert Ruiz at 1.5. The 20-year-old backstop was thrown out in his only stolen-base attempt of the season and didn't pick up a triple in 101 games for Double-A Tulsa.