Introduction to Advanced Baseball Analytics: OPS+

Mandatory Credit: Vincent Carchietta-USA TODAY Sports

Note: All statistics cited in this article are current through the games of June 6, 2022

Advances in statistical analysis have transformed the game of baseball in recent years, informing everything from free-agent contracts to defensive alignments. Driving this revolution has been a dizzying array of numerical tools, which typically share the goal of removing random chance from more traditional measures of success or failure.

For much of the 20th century, so-called ‘counting statistics,’ such as RBIs and pitcher wins, drove baseball’s numerical discourse. However, during the second half of the 1900s, and especially in the new millennium, close observers of the game began to realize the flaw of that approach: Counting stats can easily be skewed by luck or other circumstances outside of a player’s direct control. RBI totals, for example, can be inflated simply by hitting in a strong lineup or happening to come to bat with many runners on base. A seemingly more objective way to assess a batter’s skill is through On-base Plus Slugging (OPS), which has the advantage of being a rate statistic, adding on-base percentage and slugging percentage to yield a combined measure of how frequently a batter reaches base and hits for power.

OPS, though, also has its flaws. For example, as any Mets fan knows, Citi Field has typically favored pitchers since its opening, but OPS alone does not consider ballparks. Moreover, the offense has notably been down across baseball so far in the 2022 season, but once again, a hitter’s OPS does not reflect that trend. However, there is an increasingly cited and straightforward statistic that does take ballpark factors and leaguewide offense into account: OPS+, which should be one of the first tools that any baseball fan uses when evaluating the performance of an individual batter.

Let’s take a quick dive into how OPS+ is calculated and what it means.

According to the MLB Glossary, “OPS+ takes a player’s on-base plus slugging percentage and normalizes the number across the entire league. It accounts for external factors like ballparks. It then adjusts so a score of 100 is league average, and 150 is 50 percent better than the league average.” For example, in 2021, the league-average OPS was .728. So, if we ignore ballpark effects for a moment, a player with a .728 OPS would hold an OPS+ of 100, or league average, and a batter with an OPS over .728 would also have an OPS+ of above 100.

To account for ballparks in OPS+, an index called ‘park factor’ is used. The Park factor is also scaled to 100, with higher numbers indicating that a ballpark is more friendly to offense, and lower values ​​implying that it favors pitchers. In the 2021 season, Citi Field’s park factor was 98. Therefore, Mets hitters, by virtue of playing half of their games at a pitcher-friendly park, would have a higher OPS+ than a player on a team such as the Red Sox, whose home field, Fenway Park, had a park factor of 105 in 2021, the third-best for offense.

Indeed, during the 2021 season, both New York’s Pete Alonso and Boston’s Xander Bogaerts had an OPS of .863. However, Alonso’s OPS+ was 133, while Bogaerts’s was only 127.

Statistically, Citi Field favors pitchers, which makes OPS+ an important tool for giving Mets hitters their due.

Turning our attention to the current 2022 campaign, the Mets have received plenty of attention for their offensive turnaround so far this season—but the raw numbers do not even tell the entire story. The Mets as a team are second in the National League with a team OPS of .749, behind only the Dodgers, who have an OPS of .761. However, the Mets are tops in the league with an OPS+ of 115, well ahead of the Dodgers, and their 111 OPS+ in second. The difference can be explained by park factors, as Los Angeles’s home field, Dodger Stadium, has thus far been among the top ten most hitter-friendly stadiums in 2022, as have the park of two divisional rivals: Coors Field and Oracle Park. Meanwhile, Citi Field is currently exactly league average for offense in 2022.

For Mets fans, OPS+ can also open up an interested debate about the top offensive seasons in franchise history.

Unsurprisingly, Mike Piazza holds the Mets franchise record for single-season OPS, with a mark of 1.012 in 2000. While impressive, it is important to remember that 2000 was MLB’s highwater mark for offense in the modern era, with a league-record average OPS of .782 .

The 2000 season occurred during the height of the steroid era, sandwiched between the home run chases of 1998 ,Mark McGwire vs Sammy Sosa, and 2001 ,Barry Bonds, With that context, Piazza’s performance, while still historic, loses a bit of its luster. Indeed, his OPS+ that season was 155, tied for ‘only’ sixth-best in team history. Mets fans might be surprised to learn that the franchise record for OPS+ belongs to Howard Johnson, who notched an OPS+ of 169 during the 1989 season. HoJo’s .928 OPS was almost 100 points lower than Pizza’s mark, but the league average OPS was a paltry .695—more closely matching the league average so far during this season of diminished offense. In fact, given the suppression of offense thus far in 2022, Pete Alonso’s current OPS of .906 yields an OPS+ of 157, which would displace Piazza for number six on the Mets’ all-time list.

Some might be surprised to learn that Howard Johnson holds the Mets single-season record for OPS+, a normalized measure of offensive productivity.

While the normalization aspect of OPS+ helps to make it a more powerful statistic, it is not without its limitations. OPS, and by extension OPS+, can still be impacted considerably by luck.

A common instance of good luck benefiting a batter’s OPS/OPS+ is when a hitter makes soft contact but still reaches base via results such as an infield single or pop-up that falls in front of an outfielder. Fortunately, additional advanced statistics can help to flag circumstances where OPS+ is being skewed by good luck. For example, there are numerous ways to measure the quality of contact a hitter is making, such as exit velocity.

Harder hit balls generally have a better chance of falling for hits, so if a batter has consistently above-average exit velocity, it would be likely that his high OPS/OPS+ is not just a result of good fortune. Pete Alonso, for instance, has a career average exit velocity of 91.0 miles per hour, well above the MLB average of 88.4 miles per hour over that time. Unsurprisingly, Pete’s career OPS+ is a robust 140—a direct result of his propensity to hit balls very, very hard,

OPS+ is readily available on statistical websites such as Baseball-Reference and has even appeared on stadium scoreboards with increasing frequency. So, the next time you are at Citi Field, or arguing with an elder about who was the better hitter ‘back in the day,’ you might just have the perfect opportunity to impress your companions with a detailed explanation of OPS+!


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