Adam Jones started out 2009 on fire, batting .344/.400/.607 in April and May and then just .234/.297/.362 thereafter. Was it just bad luck, or did was there another reason for the decline?
Looking at the splits, I'd say it was largely a change in luck:
Jones struck out less and walked more later in the year, but his batting average on balls in play and HR/FB rate went way down. Given that his career BABIP is a still above average .318 and his career HR/FB% is an around average 11.5%, I'd say that Jones got more lucky early on than he got unlucky later.
Part of the reason behind those numbers might be his batted ball profile:
||Line Drive %
Instead of hitting line-drives and flyballs, Jones was hitting the ball on the ground quite a lot later in the year. Groundballs go for hits more often than flyballs do, but line-drive rate is the main driver of BABIP so that explains the drop there. With a more normal HR/FB rate and fewer balls going into the air in general, it's pretty clear why Jones hit 11 home runs in his first 201 plate appearances and only 8 in his remaining 318.
All that doesn't mean the Jones didn't change his approach at all though.
He started swinging more, but only at pitches out of the strike-zone. Here are the horizontal and vertical graphs of what he was swinging at, with April-May in blue and thereafter in red:
You can see that Jones was swinging a fair bit more at pitches down in the zone, which would explain the marked increase in groundballs. Pitchers may have been scared off a little by his early season power display, because it sure looks like they were trying to get the ball down against him.
In general though, there was an increase in swing rate at pitches in every direction out of the zone. What's interesting to me is that I had assumed that such a change came as a result of chasing more breaking balls, but Adam swung at curves and sliders (combined) at about the same rate. He did go after more fastballs and change-ups though, with pitchers throwing the former in the strike-zone a little less and the latter a fair bit more.
Still, Jones did end up walking more and striking out less - perhaps in part because his contact rate went up from about 74% to almost 76% - which is good in isolation. I don't think it was really the case that Adam went into a "slump" after his hot start - pitchers may have adjusted to him and he needed to adjust back, and there was definitely that big drop-off in fortune. I'd say that his final 2009 numbers - .277/.335/.457 - were pretty close to his true talent last season. Adam's still young and learning though, so I think he can take a step forward in 2010.