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A note on run differential and the deadline

Run Differential is a valuable analytic tool. But should you cite it as an absolute predictor for the rest of the season? Lets find out.

Run Utley Run!
Run Utley Run!
Drew Hallowell

So lately I've been hearing this argument bandied about, in regards to the Phillies and the trade deadline.

Chances are the Phillies won’t make the playoffs, and if they do, they probably won’t win the world series. It’s July 11 and they’re below .500. Their run scored differential is almost -50.

Now, I took that from a comment in My article from yesterday and I'm not trying to call anyone out here. But I keep hearing this argument over and over again, whether it's someone commenting on twitter, or coming from very educated, smart, statistically minded writers, with strong understandings of saber-metrics and the team.

What many people are adding to that basic argument, that the comment doesn't allude to, is this:

Since the Phillies have a -50 run differential, they are playing above their pythagorean record and will regress, or at best stay the same, and have little chance of making the postseason.

Now I've never been a fan of run differential as a mid season predictor, there are just too many variables in my opinion, but I felt I owed it to myself to, you know, actually make sure I was barking up the right tree before I made the argument for or against.

If I had the time what I would have liked to do, was go back through every season, since 2005 and look at run differential, for every team, from around the middle of July both before and after. That would have taken a month.

Instead, I just isolated 2012 for the national league.

And here's what I found.

  • At the 90 game mark, the 2012 Phillies had a run differential of -23. Over the remainder of the season, without Hunter Pence and Shane Victorino, and later Joe Blanton, They went 42-30 with a run differential of +29. Must be an anomaly, right?
  • Well, of the remaining 8 teams that at the 90 game point had - run differentials, 4 of those teams (5 total) went on to have positive run differentials the rest of the way. The Phillies +29 was the lowest total of any of those teams.
  • There were 7 teams with positive run differentials in the first 90 games of 2012. 3 of those teams showed a negative run differential the rest of the way

So to recap:


How could this happen? Well, basic statistics will tell you it happened because:

  1. Players changed teams.
  2. Teams played different teams
  3. strength of schedule
  4. the variables changed for every team.
  5. the first 90 games were only a small sample of the whole.

If you think about it, run differential as an indicator is flawed at the half point. If you believe that run differential is correct right now (and it might be), to use it to accurately predict or examine the rest of the season, you have to also assume that

  1. The Phillies will play no better offensively, defensively, or pitch any better.
  2. The team will not change personnel in any way
  3. The teams you FACE will be no better or worse
  4. Those same teams will play exactly how the teams you already faced played.

The reason run differential swings so dramatically at the mid point of the season, league wide, is because of the trade deadline. Teams change. Rosters change, people change.

Run Differential is a fantastic tool to analyze past performance. It's a tool you use to determine how to improve your club for sure, and it can be an indicator of what you would expect to do if you don't do anything, but it is in no way an absolute indicator of future performance. Especially in an environment where the variables are changing.

When the variables CHANGE, you adapt to the variables.

Now I don't claim to be a statistician. I understand the basics, so it amazes me when people smarter than me can cite things as absolutes that are not. Sometimes this comes from taking the statistics to BE absolute, and sometimes it's just lazy, whatever. But the reality is this: If you cite Run Differential from the beginning of the season to today, as an ABSOLUTE INDICATOR of anything, you're barking up the wrong tree.

One last thing on odds, and playoff chances.

In 2011, we screamed DON'T BUY!! when the Phillies had a high chance of making the postseason, and we all said this because we know the postseason is a crapshoot.

Those of you who are making THIS argument as a reason to sell, are in a lot of ways doing exactly what you (rightly) warned against at the time. The odds argument (The Phillies odds are low, and if they get there they'll play better teams) Is a fancy way of saying

"We might not make the playoffs and the playoffs are a crapshoot, and we don't have a good enough team to win anyway, so lets sell everyone off"

But this team's goal EVERY YEAR is to make the playoffs. That's who they are.

So if the goal is to try and make the playoffs?

Then the odds say you stand pat, and take your chances, or you buy and make a run. But they in no way tell you to sell right now. At least not valuable assets.

You can't have it both ways. The odds actually say that to MAKE the playoffs, to HAVE A CHANCE, the Phillies need to get better.

So if you're going to make the odds argument, and you're against buying, for whatever reason, (and I'm not in any way advocating that being against buying is wrong. You are entitled to that opinion, and there is a very strong case that can be made NOT to buy.) make the argument to stand pat. You'll make more sense.