Are the players of today better than players of the past? If so, how much better? These are not easy questions to answer. It is possible to construct a study to estimate it. David Gassko did such for The Hardball Times two years ago, and did a good job, producing very reaonable results. Unfortunately, it is far from conclusive and very sensitive to whatever assumptions you use to set it up.
The approach, which I have played around with, is to compare how players do from one season to the next. But how much of it is age decline, and how much is the change in league strength? You really can't tell, since the same process we're using here is what we use to compute aging factors in the first place.
It is necessary to regress to the mean as well. Take two average players, but one is +15 by luck and the other is -15 by bad luck. The next year, player 1 should be average. But player 2 may not play, or else be limited to a bench role, so as a group you would see decline even if there was no real change at all.
Some criticized David's study for regressing too much, including some Baseball Prospectus Authors, who apparently didn't see the need for regression at all. They are demonstrably wrong about not needing regression*, but may be right in that David did too much. It's pretty much impossible to tell.
I tried to get around the regression issue by choosing only players who were average in year one, or very close to it. Even with that, what age ranges should I include? I tried using ages 26-28 in year one (27-29 year 2). This would work if it were true that on average, 28 year olds were equal to 27 year olds. I also tried looking at ages 25-28 and 26-29, which would look at, on average, ages 26.5 and 27.5. This works if age 27 is indeed the true peak. The difference in results is tiny, but become huge when chained to look at seasons 100 years apart.
Another issue that isn't even touched is improvements that affect all players. We're assuming that the player is constant from year 1 to year 2 and any changes measured represent the league changing around him. What if some new, illegal or legal, nutritional supplement makes every hitter and pitcher better? This study would not pick that up at all. I'm not sure it even matters. If there are great advancements making everyone a better player then we should not penalize great players from the past for not having these advantages. If they played today, they would have them just like everyone else. I think we want to look at the greatness of the player, not what improved circumstances allow him.
One thing that can should be considered is game improvements from using a greater pool of the best available players. Major Leaguers before 1947 would not have dominated their leagues to the same extent if they had not excluded players of darker skin color.
*Honus Wagner is the case in point. The no-regression time lines would make him a replacement level hitter (and Ty Cobb not much better I guess) in today's game. Here are some factors that determine how great a ballplayer is, in order from the ones that seem to have changed the most over the years to the least:
1. Size and Strength
3. Throwing Ability
4. Hand/eye coordination, reactions.
No question that players of today are much bigger and stronger than players 100 years ago. But Wagner was an exception, 5'11, 200 pounds, and was one of the earliest players to work out with weights. He would not be out of place in today's game. I'm not sure how his speed would rate today. His arm would be one of the best among shortstops, as Wagner is known to have made some incredible long distance throws, I believe near 400 feet. I don't think it is possible, even under the most favorable circumstances, to throw a baseball that far without being able to throw at least 90 MPH. I can't think of any reason why #4 would change that much, unlike muscle mass and speed throw better nutrition. Wagner was the best of his day in this regard, and would almost certainly at least be among the best today.