Addressing the Tigers initial move first, we will compare Sean Casey and Carlos Guillen's defensive abilities at first base using some alternative metrics to those used by Paul. In doing this, I echo Paul's belief that any ratings of Guillen's defensive abilities at first base are at a minimum suspect given the relatively small data set he has provided to date. However, in situations such as this, we have to work with what is available.
Since the initial debut of Zone Rating, the formula – while controversial – has been revised by John Dewan to a format that I believe gives a more accurate breakdown and holistic representation of what we can expect from an individual at a given position. The Revised Zone Rating (RZR) not only removes extra credit that had previously been awarded for turning double plays, but also removes those plays which a fielder makes outside of their defined zone; instead considering those as a separate statistic defined as Out of Zone (OOZ) rating. Further details and full statistical rankings can be found at HardballTimes.com.
When looking at these additional statistics, we find that in 2007 Sean Casey posted a .707 RZR, ranking him 16th among qualified first basemen (NL and AL combined). In contrast, during Guillen's 178 innings at first base in 2007 he managed a .906 RZR. While it is unlikely Guillen would have been able to maintain this rate over the course of a full season given the fact that the league leader (Albert Pujols) finished with a .843 RZR, there is little doubt that he would turn a significant number of additional balls within the first base zone into outs when compared to Casey. If we look back in Casey's history, his performance in 2006 (though limited to 413 innings) gives the impression of a precipitous decline in performance as he scored a .919 RZR; though a closer look shows that in 2005 and 2004 he was much closer to the relatively poor defensive performance of 2007, notching RZRs of .771 and .779, respectively.
Looking at the additional component generated by the reworking of the Zone Rating formula – Out of Zone rating, we find additional interesting information. In 989 innings at first base in 2007, Casey managed to convert 22 balls outside of his zone into outs. That figure once again ranked him 16th among qualified first basemen. Given that this statistic is directly related to playing time at a position, it is difficult to compare Guillen's 2007 results. However crude the translation may be, we can make the bold assumption that Carlos may have kept up the pace he set over 178 innings of action (an OOZ rating of six), which would have resulted in 33 balls outside of his zone converted into outs if given comparable innings to Casey. Even if he was unable to maintain his pace, Guillen likely would have rated somewhere in the upper third of the league in this category, putting him ahead of Casey yet again.
Another metric that can be considered when comparing the two players is the Probabilistic Model of Range (PMR) developed by David Pinto of BaseballMusings.com. The essence of the model is a calculation that gives the probability that a batted ball will be turned into an out based on six parameters; 1) direction of the hit, 2) the type of hit, 3) how hard the ball was hit, 4) the ballpark, 5) the handedness of the pitcher, and 6) the handedness of the batter. A full, detailed description of the model can be found here.
While Guillen didn't have enough balls in play during his time at first to qualify, we can still make some rough determinations about how he may have rated in the system. In 2007, Casey scored a ratio of 93.56 under PMR, where an average score is 100. In a very simplistic view, this basically means that Casey didn't turn as many balls into outs as the model suggests the average first baseman should have, given the balls in play Casey experienced. As a team in 2007, Tiger first basemen accounted for a 95.44 PMR score. This indicates that the combination of Carlos Guillen, Marcus Thames, and Mike Hessman actually improved the team's overall PMR rating at first base. Chalk that up as another general plus in favor of Guillen, though a slim one given the uncertainty of how much he actually contributed to that increase.
Moving on to a comparison of Guillen as a shortstop to that of Edgar Renteria, I believe we find additional encouraging results. Guillen scored an RZR of .801 and an OOZ of 36 in 2007, compared to Renteria's .815 and 49. Similar advantages for Edgar Renteria can be found in looking at the same data from 2004 through 2006. In short, these metrics indicate that Guillen is unlikely to convert as many balls in his zone into outs as Renteria, and he is also unlikely to convert as many balls out of his zone into outs as Renteria. Given this information, it seems quite clear that Renteria – in spite of his perceived and real decline as a defensive shortstop – is still the superior defender to Carlos Guillen.
Taking a peak at PMR for these two players, Guillen scores a 95.33 versus Renteria's 98.87. While both players are credited with being below average defensive shortstops by PMR, Renteria again outpaces Guillen in this metric. The 2007 data represents a switch from previous year's data, as Guillen had rated as the better PMR defender in both 2005 and 2006.
The above defensive metric comparisons of Guillen and Renteria appear on the surface to cloud the issue a little; leaving open the possibility that it may not have mattered much which player was at shortstop. However, if we look at one additional factor, I believe it may become more obvious that we are significantly better off with Renteria manning the infield's toughest defensive position instead of Guillen. That additional factor is the third baseman. With Brandon Inge playing everyday alongside Renteria, I believe his slight advantage over Guillen will be exaggerated; giving the left side of the Tigers' infield a significant boost.
In 2007, Renteria had Chipper Jones alongside him most of the time, and while Jones certainly isn't a horrific defender at the hot corner, I think it is readily apparent that he is not in Brandon Inge's defensive class. Just in case it isn't apparent to everyone, let's run a comparison using some of the same stats mentioned above, as well as John Dewan's Plus/Minus System from his book The Fielding Bible. In looking at RZR in 2007, Inge managed a .712 against Jones' .662, while Inge's OOZ score of 63 bested Jones' mark of 57. A wider margin is evident when comparing their 2006 numbers as Inge posted RZR and OOZ scores of .780 and 49, against Chipper's ratings of .640 and 34. Inge's greater ability to turn balls out of his zone into outs should help curb some of the deficiencies seen in Renteria's recent defensive performances.
As one may expect given the previously mentioned numbers, Inge also significantly bests Jones in terms of Plus/Minus. During the 2007 season, Brandon scored a +22; good enough for second in the league behind the Giants' Pedro Feliz (+27). That +22 score indicates that Inge converted 22 more balls into outs than the average third baseman over the course of the season. Chipper Jones didn't even make Dewan's top ten list. You will find similar results in 2006 as Brandon Inge led baseball with a +27 rating at third base, while Jones again did not make the top ten.
When you consider all this information in totality, the evidence of a substantially improved infield defense becomes quite obvious. Recapping, you are replacing Sean Casey's abilities at first base with a player (Carlos Guillen) that the data suggests should convert more balls in the first base zone into outs, while also converting more balls outside that zone into outs. You are replacing Guillen's declining defensive skills at shortstop with a player (Edgar Renteria) who was more proficient in converting outs both within and outside the shortstop zone. In addition, while Renteria's defense considered in a vacuum may only represent a slight improvement over that of Carlos Guillen, if considered in conjunction with the dramatic difference in defense at third base between Chipper Jones and Brandon Inge, there is increased reason for optimism on the left side of the infield.
While Paul has correctly suggested that the changes at first base and shortstop that the Tigers have opted to make may not create an ‘iron wall' around infield, I believe their 2008 defensive infield will be closer to that of an iron wall than it will be to that of the 2007 Tigers.