This isn't to say he's been perfect. He's struck out a lot--24% of plate appearances--and walked infrequently--5% of plate appearances--over the past two seasons. The National League averages are 20% and 6%, respectively. So he stood to improve on his plate discipline. But, as Travis Sawchik of the Pittsburgh Tribune pointed out at the end of spring training, Marte improved markedly in the second half of last season, cutting his strikeouts and increasing his walks. Among players with at least 200 plate appearances after the All Star break, Marte's .408 on base percentage was second in the National League. So he entered the 2015 season with high expectations.
It hasn't worked out that way so far. Granted, we're only seven games into the 2015 season, and a lot can change. But to this point, Marte has struck out in 47% of his plate appearances, the highest percentage in the National League. (Teammates Gregory Polanco and Pedro Alvarez rank second and fourth, respectively.) How bad is that? You may recall Cubs prospect Javier Baez, who hit seven home runs in his first 19 games following an August callup last season. He didn't make the club this year, as his free-swinging ways yielded a .169 batting average and strikeouts in 41% of his plate appearances. Marte's .115 batting average is lower and his strikeout rate is higher this year than Baez's figures were last year. What's more, Marte's walked in just 3% of his plate appearances, tying for the 16th lowest rate in the league.
So what's going on? This isn't like a long hitless streak, which can be partially dependent on luck. Sometimes you scorch a line drive and it gets caught. Sometimes you hit a weak pop that falls in. Strikeouts, though--unless you're getting routinely jobbed by the umps, that's on you. So let's eliminate bad luck as an explanation.
Are pitchers treating him differently this year? It's early in the season, and so sample sizes are small, but here's the breakdown of pitches the Marte has seen, courtesy of Brooks Baseball:
Maybe he's being pitched differently. Here's where he's seen pitches this season. This is from the catcher's point of view, so imaging Marte standing the right of this diagram.
This diagram is called a "heat map," and the colors correspond to frequency. Red areas are "hot," blue areas are "cold." As you can see, pitchers have been throwing a lot of pitches down-and-in, outside the strike zone (11% of the pitches he's seen) as well as in the down-and-out corner of the zone (8%). Is that different from last year? Here's are the 2014 data:
There really isn't a whole lot of difference between these heat maps. Pitchers are throwing to about the same part of the plate. So why's he striking out so much?
Two reasons. First, he's swinging at pitches that he laid off last year. Per FanGraphs, in 2014, he swung at 66.3% of the pitches that were thrown to him in the strike zone. This year, he's swinging at 66.7%, That's an immaterial difference. The problem is outside the strike zone. Last year, he swung at 36.6% of the pitches he saw outside the zone. This year, he's swinging at 45.2%. That's a huge difference. Among 65 batting title qualifiers last year, Marte was 11th in the National League at swinging at pitches outside the strike zone. This year, he's sixth.
Now, swinging at pitches outside the strike zone isn't necessarily bad. Pablo Sandoval is a notorious bad-ball hitter. He swung at 45% of pitches outside the strike zone last year, the most in the league, and he was a successful hitter nonetheless. The reason is that he made contact. Sandoval's contact rate on outside-the-strike zone swings was 80%, the second highest in the league. As a result, even though he swung at everything, he was, amazingly, the ninth-hardest batter to fan in the league last season, striking out in just 13% of his plate appearances compared to a league average of 20%.
Marte's not Sandoval. Here is a side-by-side look at his whiff rates, i.e. the percentage of swings on which he misses the ball, in 2014 (at left) and 2015 (at right):
You can see the problem. At left, his whiffs per swing heat map for 2014 is fairly "cool"--a lot of blue, some light purple. The colors are a lot darker and redder in the heat map at right representing 2015. That means he's swinging and missing a lot more. For example, look at the bottom row of each heat map. Those are low pitches, below the strike zone. In 2014, Marte swung at 274 of them. He made contact 146 times, meaning that he missed on 47% of his swings. This year, he's swung at 16 pitches that were below the strike zone, and he's missed all but three of them--an 81% whiff rate. That's the problem. In 2014, he whiffed on 15.3% of his swings at pitches in the strike zone. This year, he's at 19.0%. That's not great; he's slipped from ranking 75th among 101 players with 400 or more plate appearances in 2014 (26th percentile) to 76th among 95 batting qualifiers this year (20th percentile). But on pitches outside the strike zone, he whiffed on 40.9% last year, and this year it's 60.7%. He's slid from 73rd (28th percentile) in the league to 88th (7th percentile).
Here's the point: His problem is pitches outside of the strike zone. He's swinging at more of them this year than last year, and missing on a lot more of those swings. Maybe he's being overly aggressive. Maybe he needs contact lenses. I can't tell. But it's very clear that unless Starling Marte lays off pitches outside the strike zone that he can't hit, his strikeouts are going to limit his offensive production.
Obviously, analysis like this would be impossible without websites like FanGraphs and Brooks Baseball.