Have you noticed that Jason Kendall isn't hitting very well this year? And that Jack Cust is? Yeah, me too. Let's suppose for the nonce that Cust doesn't turn back into a pumpkin, or some other variety of large squash, and keeps hitting enough that his spot in the lineup is a given. Whither Mike Piazza? Could he be the A's every day (or every other day) catcher, or is his defense really so bad that Jason Kendall (ugh) is still the better option.

Here's what the numbers say:

Defense:

I'll start by comparing Kendall's defense to Piazza using only last year's numbers:

                        Inn     SB      CS      E       PB      A
Kendall 2006    1254    71      31      5       7       54      
Piazza  2006    718     97      13      8       7       34
Piazza (proj)   1254    169     23      14     12       59

Piazza threw out runners at a much lower rate than Kendall, and the opposition further took advantage of his weak arm by running more frequently. All told, projecting Piazza's numbers to a full season, he would allow an extra 98 stolen bases, with 8 fewer caught stealing. In addition, Piazza would be responsible for 9 extra errors and 5 extra passed balls.

Using the run values from Tom Tango's work on Base Runs, this is worth approximately:
98*(.198) + 8*(.282) + 9*(.481)+5*(.276) = 27 runs.

That is, I'd expect the A's to allow about 27 more runs over a full season with Piazza as their everyday catcher than with Kendall. And that doesn't include any contribution from range in fielding bunts or other balls around the plate, blocking balls that would be scored as wild pitches, or any supposed effect that Kendall's game calling has on the pitchers' ERA.

Before we move to offense, let's look at how Kendall is doing so far this year, and again project it to a full season:

Kendall 2007    346     33      9       2       6       18
Kendall (proj)  1254   120     33       7      22       65

Using the same formula, compared to last year, Kendall has slipped on defense to the tune of
49*(.198) -2*(.282) + 2*(.481) + 15*(.276) =
14 runs/year, or about half the difference between him and Piazza last year.

Offense:

As I'm only trying to get a rough estimate here, I'm going to use OPS, a simple but crude tool, as a way of measuring offense. There are better ways of doing it, but I'm sure the difference wouldn't amount to more than a couple of runs. Predicting offensive performance is still difficult; putting a run value on that performance once it's happened is much easier, and for our purposes using OPS is good enough.

Let's again start by (mostly) ignoring what Kendall has done in the first couple of months of this season, and look at the previous years instead. In 2005 he had an OPS of .666, and in 2006 .709. Let's suppose he's declined a bit from that, but not to the depths of his 2007 performance, and say he could be expected to have a .650 OPS.

As for Piazza, he's also off to a fairly slow start (.718 OPS) this year, but was at .843 last year, .778 in 2005, and .806 in 2004. So how about a predicted .800 from him.

Now, over the course of a season, each point of team OPS translates into about 2.15 extra runs scored. Supposing that an everyday player gets about 1/10 of his team's plate appearances, this would give a difference of
150 * 2.15 /10 = 32 runs

If we instead use Kendall's actual performace this year (.435 OPS) as the basis of comparison ... well, I'll leave this one as an exercise for the reader. It's too ugly for me to contemplate.

There's one other point worth noting. Because teams are more likely to steal in close games than in blowouts, the extra runs scored will be more highly leveraged than average. So comparing runs saved with the glove to runs produced with the bat probably undervalues the better defensive catcher when it comes to wins and losses.

Conclusion:

Based on their performance over the last couple of years, we can conclude that Kendall and Piazza are surprisingly close in overall value at catcher. What that means for this year is still open for debate.

If you think that Kendall can rebound to his established levels of good-but-not-great defense and passable-but-not-good offense, and you also believe that his game-calling has a tangible effect on the pitching staff, or that Piazza's offense would suffer if he were moved back behind the plate, then it would make sense to stick with Kendall.

On the other hand, if you think that Kendall's awful start is a sure sign of a real decline in abilities compared to what he could do over the last two years, then it's worth tolerating Piazza's awful arm for the upgrade in offense.