This process could only fairly be represented by several statistics
1) Percent of works submitted that are taken on
2) Odds of sale within one year if a work gets taken on
3) Odds of sale, ever, if the work gets taken on.
4) Average return on successful sale
5) Average client satisfaction with agent after X years or termination of contract, whichever comes first.
These are all statistics, and with them, one could make a pretty nice judgment. Sure, *one* statistic is grossly unfair, but having the whole collection will let you tell Binky Urban from Jane Noname.
Ok, let's take a look at what you'd need to generate those numbers:
1. To get a percentage of works taken on to works submitted, you'll have to measure both things. Let's talk about works submitted. Are you going to count only the people who mail me letters? What about the nitwits who send me equeries; do they count?
What about the people who don't include an SASE; do they count? Or the people who send a genre I don't represent; do they count? And the people who don't send pages and are asked to do so; do they count as a query or a partial? And what about the people I approach? How do you count them?
2. When does the year start? From the date you query me? From the date I read it? From the date you finish revisions? What if sell it and the deal falls through; is that "a sale"?
3. What about the book I sell that came from an idea the publisher gave us when I queried on your novel? He never buys that novel, but he buys the other. Does novel get counted as "never being sold?" even though the other work wouldn't count as a query?
4. Do you mean the advance? What if I get you a great royalty percentage with a lower advance? What if you don't want a big advance so you can earn out and get a multiple book deal? Does that count against "my stats"?
5. Well, the only thing to do here is quote the Rolling Stones.
And even if you could agree on what would be measured and how, here's the dogs honest truth about why no agent would do so: the only people asking for this are writers. And I know this is not fun to hear but it's the truth: you're a glut on the market.
I'm not going to invest ANY time or money in compiling stats to attract authors. I attract enough as it is, and I don't want to attract people who think you can correlate submissions to sales and effectiveness. The more effective the agent, the less they need to attract you. The only people who would benefit from this are frauds and scam artists who could say "we scored 100% on the Agent Effectiveness Benchmark".
If you think I'm blowing smoke, go check out the ICM website. They are the biggest agency in town, and I'm sure they close a lot of deals and high percentage of deals from work they take on. Under "contact us" they tell you as plain as can be-don't. They don't waste a single letter telling you they are big deal. They know it, you know it, and I know it. How do you know it? It wasn't cause you saw any damn statistics on their sales.
There are no guarantees in this biz. There are no across the board benchmarks. There is only the old reliable "what have you sold", and the remorseless teacher called experience.