Last year, I decided to start logging my own MLB money lines from several sports books, as this seems to be the hardest piece of "historical" information to find. I started logging in mid June 2008 through the end of the season.
Right now I'm looking at the difference between opening and closing lines. I have 3 different metrics that I am using to evaluate the performance of any particular predictor (that predicts a win% for each team). You can read more about the metrics I used
here.
I have 1309 games logged from last year. I used the winning percentages "predicted" by the lines as an input to each of these metrics. Depending on the metric, the opening lines show an improvement of 2.5 - 3.5% over simply assigning 50/50 to each team. This number seams reasonable to me.
However, what seems surprising is the closing lines. The closing lines show only 0.1% - 0.15% improvement over the opening lines in terms of their predictive power. I would have expected a much larger improvement in predictive power from the closing lines.
So, what do you think? Do these numbers make any sense? Is my data bad? Are my metrics bad? Or, are
closing lines really not much more "powerful" as a predictor than opening lines?
**One thing to note, I only logged lines ~10 times per day, so my data won't show any motion that happens either right after the line comes out or right before game time.