Over the past couple of seasons, Ervin Santana has been one of the loan bright spots in an organization that is starved for starting pitching. In 2016, Santana put up the Twins’ lowest ERA from a qualified starting pitcher, at 3.38, since Johan Santana’s 3.33 in 2007. He followed that up in 2017 by improving his ERA to 3.28, the 5th lowest in the American League. This was a big turnaround from a pitcher who had an ERA of 4.16 over his 11 career seasons prior to 2016.

So, what is the reason for Ervin Santana’s success over the last couple of seasons? Well, it is quite simply his slider, or more specifically the usage of his slider.

Throughout his career, Ervin Santana’s slider has always been considered his best pitch. However, since his slider isn’t a big wipe-out pitch, like those of a Max Scherzer or Chris Sale, it has never been considered to be one of the best in the game.

According to Fangraphs’ Pitch Value metrics, in 2017, Ervin Santana’s slider measured out at 1.83 runs above average per 100 sliders thrown. This was the 6th best number by a qualified starting pitcher last season. Max Scherzer lead the way at 3.33 runs above average, more than a full run over the nearest pitcher.

The Pitch Value metrics weren’t the only measuring system that liked Ervin Santana’s slider. Again, among qualified starting pitchers in 2017, Santana allowed the 5th lowest wOBA on his slider at a mere 0.211 (MLB average wOBA in 2017 was 0.321). Again, Scherzer was way ahead of the pack, posting a 0.138 wOBA allowed on his slider.

If you prefer to look at more traditional statistics, Santana allowed a 0.162 batting average against on his slider last season, which ranked 6th in Major League Baseball, and of the 31 home runs Ervin Santana allowed last season, just 8 of them were off of his slider.

As you can see, the numbers clearly back up the fact that Ervin Santana had one of the best sliders in Major League Baseball in 2017, and if you go back beyond that you will see that this has been the case for nearly all of his career. So, what has made the last couple of seasons different than the majority of the first 11 seasons of his career? Quite simply, it has been the vamped-up usage rate of his slider.

In 2015, Santana’s first season with the Twins, he threw his slider on 33.7% of pitches, on his way to a 4.00 ERA. However, in 2016 and 2017, he increased his slider usage rate up to 36.8% and 36.5% respectively. This increase factored in over the course of a full season, adds approximately 100 extra sliders that Santana is throwing instead of either his fastball or change-up, which are much worse pitches for Santana comparably.

While back-to-back seasons of increased slider outputs coinciding with improved pitching by Santana is noticeable, it is hard to say that this is an established trend. So, I decided to look back at Santana’s numbers throughout the course of his career to see if this pattern has always been the case, or if maybe it was just a coincidence.

In the chart listed below, each of Ervin Santana’s 13 career MLB seasons are ranked by slider usage rate and matched up with his ERA in that season to see which seasons had the lowest ERA comparatively. Along with that, is a linear model that illustrates the correlation between Ervin Santana’s slider usage rate and his ERA.

When looking at the linear model, we can see that there is indeed a negative correlation between Ervin Santana’s slider usage rate and his ERA. This means that as his slider usage rate goes up, his ERA goes down. As is almost always the case with data, it would be nice to have more data points to reference in order to gain an even clearer picture of the effect Santana’s slider has on his ERA, but 13 seasons of this being the case is still pretty strong evidence.

In the chart, we can break down some of the numbers even further. In each of the five seasons where Santana used his slider the most, he had one of his six lowest ERAs of his career, including each of the top four. We can also see that the three seasons where Santana had the lowest slider usage rate were also the first three seasons of his career. This is a result of Santana ditching his curveball that he threw early in his career to start focusing more on his slider.

Now, to say that Ervin Santana’s slider usage rate is the end all be all towards his success would be foolish. There are many factors at play when it comes to the success that he will have in a given season, but for Santana, it appears that his slider usage rate is one of the more important individual factors when it comes to determining his success.

So, what can the Twins take away from this? Obviously, Santana can’t begin to just throw his slider on every pitch, as opposing hitters will adjust, and make his slider less effective. There is also a point where if Santana throws too many sliders, it will wear on his arm. I think the best approach would be to try and increase his slider usage by a couple percentage points to start the season and see what kind of effect this has on Santana’s performance.