Left on Base Percentage (LOB%) measures the percentage of base runners that a pitcher strands on base over the course of a season. This stat does not use the left on base numbers reported in box scores, but instead is calculated using a pitcher’s actual hits, walks, and runs allowed results:
LOB% = (H+BB+HBP-R)/(H+BB+HBP-(1.4*HR))
Most pitchers have LOB%s around league average (which is approximately 70-72%, depending upon the season), and pitchers that deviate from that average tend to see their numbers regress towards average in the future. In other words, if you see a pitcher with a 60 LOB%, they are letting lots of runners score so their ERA will be high, but the odds are that they will strand more runners in the future and lower their ERA.
Not all pitchers will regress toward league-average, though: high strikeout pitchers have been shown to have some control over their LOB%. Pitchers that record a high numbers of strikeouts can pitch their way out of jams more easily than pitchers that rely upon their team’s defense, so they are able to maintain LOB%s higher than league average. Also, if a pitcher isn’t a major-league caliber starter — or if they’re a borderline case — it’s likely that their true-talent LOB% is below league average.
By using this statistic in conjunction with others — specifically BABIP and HR/FB — it’s possible to get an idea of if a pitcher is under- or over-performing and likely to regress. For more details, see the two videos below: