Strand rate, or left on base percentage (LOB%), measures the percentage of baserunners a pitcher leaves on base over the course of an entire season. At its core, strand rate is simply a measure of the number of runners that could have scored, but never actually did. Here's the formula:
LOB% = (H+BB+HBP-R)/(H+BB+HBP-(1.4*HR))
Note: A pitcher's strand rate is NOT calculated using the actual baserunners left on base at the end of an inning, as you would see in the box score.
Over the last five seasons, the typical league-wide strand rate is usually around 73%. In other words, a typical baserunner scores just 27% of the time. Pitchers who post rates above 73% are generally considered fortunate, while pitchers who post rates below 73% and usually considered unlucky. In both cases, normalization towards the league average is generally expected.
As we've developed more metrics and have become more sophisticated with our analysis, we've determined that some pitchers have more control over their numbers than others. For example, a pitcher with an elite strikeout rate is more likely to be able to pitch themselves out of a jam than one who has a high contact rate allowed, since they are more like to escape an inning with a strikeout. As such, pitchers with elite strikeout rates tend to also have high strand rate baselines, meaning instead of comparing a pitcher to the league average, it's generally preferable to compare him to a career baseline (where a reasonable sample size is possible).
The stat itself is highly variable, which is why it's such an important one to use in conjunction with BABIP and other quality of contact metrics when evaluating a pitcher's performance. A big reason for this fluctuation is because there are a myriad of factors that can change the outcome - ranging from the quality of his bullpen, ballpark factors, the quality of the defense, umpire inconsistencies and most importantly, play sequencing. Remember, LOB% should be utilized as the starting point, but your conclusions should be drawn from the indicator stats.
Here's a glimpse at the players with the lowest LOB% and highest LOB% from 2019, which we'll use to identify those who have the best chance at realizing improvement or degradation to their numbers, respectively.
Starting Pitchers With >100 IP in 2019
As you can see, it's important to realize that there's no silver bullet metric and just because a player's strand rate may be out of line, you shouldn't draw conclusions until looking through all the numbers. For example, Gerrit Cole's 83.3% from 2019 is on the high side, but his career strand rate baseline of 75.6% already indicates that he usually performs above league average. When you also factor in his 39.9% strikeout rate vs career strikeout rate of 27.6%, it's evident Cole has not only made a change in his pitching approach, but this change allows him more control over his strand rate than others on this list. Strand rate is nothing more than a window which we can use to better understand and rationalize pitcher performances.