Just Wondering

So I've been wondering about something, and I haven't really been able to think of examples to come up with a working answer to the question, which is: How accurate a predictor is a writer's first book to the body of work that follows?

Put another way, if an author's first book is a mediocre read, how many "improve" and produce excellent ones? I know of many that work the other way around (as I've blogged on and on about), where their first book or two are wonderful and then they slide into a continual repetition of their own formula.

At conferences I've heard it said that in the good old days, the average writer's first published novel is actually the second or third they've written. Nowadays it's supposedly their fifth or sixth, the point being that publishers aren't as willing to take on new talent and nurture it as they once did. Does this have an effect on the question?

So I leave this to anyone who may stumble across this and humbly ask for comments. What do you think? And do you have any examples?