Dear Lou,
I enjoyed your article about the value of analyzing splits to make pitching line-up determinations. I have a couple of questions about it.
Thanks for the kind words! Fire away... (Note: That was word for word! )
1) How much of a sample size is necessary for the split to be meaningful? (He cites a 21 AB sample)
I think your premise for the question is flawed and as such, it can drive you crazy.
I wouldn't look at any sample being as meaningful and not meaningful. A baseball date sample is not a zero-sum argument. Instead, I would look at it as "more" or "less" meaningful.
In your example 21 ABs is not a sample I'd bet the house on but it's not throw away either. I wouldn't lean on 21 ABs too heavily in almost all cases, but if that sample combines with other data to support a trend or theory or even an established model, I might look at it differently.
For example knowing that a left-handed hitter likes to go with power to LCF, if I see he's had 21 great ABs at Fenway, I can put apples and dumps trucks together and give that little more weight than if I just have 21 ABs with nothing to back it up. I can theorize that Fenway fits his offensive style well and those 21 ABs support that ...
Now, for example, if Jimmy Rollins hits 11 of his 15 HRs in a year in 21 ABs at RFK, I might look at that sample differently, knowing what I know. In that case I am probably thinking "aberration" until more ABs prove me wrong.
And keep in mind that I have to take 21 ABs and make a guess. That is my job. I make a living getting to the truth before the average guy ... or more accurately that's what I try to do. You don't have to, although that skill is useful in competition, so you don't have to lean as on heavily on small samples.
I guess the short answer here would be there is no "truly meaningful" answer ... 7 ABs has little meaning ... 21 has more meaning ... 210 has a lot of meaning ...
All of these things are puzzle pieces ... They all relate to the big picture ... If what you are looking at is an all white piece, that doesn't help you see what the puzzle is a picture of, then it is of little help. But If that piece is the black nose that tells you it's a polar bear in a blizzard, it doesn't matter if the piece is small. It still reveals a lot.
Use any sample, but generally the smaller it is the less you lean on it.
2) Fantistics has always endorsed the weekly line-up rater as a method to decide between two players of similar caliber. How does the weekly line-up rater reconcile with the idea of using splits in making line-up determinations? So say I have a lefty hitter whose splits suggest that he won't do well facing lefty pitchers. And he will be facing 4 lefties in the coming week.
But according to the weekly line-up rater, the pitchers he will be facing in the coming week have not done well recently, so his rating is very high.
On the other hand, I have a hitter of similar caliber whose splits say he is a good early season hitter. However, his number on the weekly line-up rater is low because the pitchers he will be facing have done well in the last few weeks.
Which player would you choose to use in this situation? Or put another way, which tool is more reliable in making line-up judgments when they are telling me different things?
Again, think pieces of a puzzle. And again, the premise of the question is flawed. A doctor has all kinds of tests at his disposal. An MRI isn't "more reliable" than a blood test, but it sure works better if the question is whether your elbow ligament is strained or torn. Nevertheless, it won't do you any good if there are issues with your cholesterol level.
The Lineup Rater takes one thing into account ... what that hitter's scheduled opposing starters for the week have given up for OBP lately ... or in the case of pitchers, what their scheduled offensive opponents have posted for OBP lately.
When considering a choice between two players of equal caliber this is a good thumbnail starting point, but it only looks at one factor. One guy's opposing starters may be cold but they still may own that hitter historically. You have to understand that the Lineup Rater is one piece of the puzzle, a handy way of helping you make your initial decisions about which guy to start. There are a number of ratings like this, or splits, that would be helpful, but it is similar to sample size. The more you take into account the higher the level of "reliability" ...
In your example, working just on what you gave me, I lean more heavily on the fact that first hitter can't hit lefties because that's what he's facing. If his opposing pitchers are cold there could be any number of other factors involved there, for example his last three starts has been against the Red Sox, Yankees, and Blue Jays on the road. That relates less to your hitter than does his problem with left-handed pitching.
If you are convinced the second guy's early season success is a characteristic of the player and not just the result of a good two weeks two years ago, then I would go with him because his split relates to his abilities more than the OBP of his opposing starters against other teams. His split is more relevant ...
But both are useful. Both have meaning. But in any given situation, some indicators, and some sample are more useful and more meaningful than others. The trick is to figure out which is which.
Hey if it were easy anyone could do it! <g>...
In the end it is a matter of experience. You do it for a while and find your own level of faith in all the numbers that are available to you. Ask around our staff and I bet we all have varying levels of confidence in any given stat and that confidence is fluid depending on the question, and the player in front of me. If you look at these things enough you'll make some good decisions and bad ones but you'll start developing faith in the indicators that work well for you, the tools that fit the job in front of you.
I'll bet they'll end up a lot like ours.
Hope that helps ...
Thanks for the discussion and most importantly thanks for being one of us here at Fantistics!
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