Monday, February 17, 2014

What Positions Do Teams Value in the Draft?

Abstract
Where players are taken in the NFL draft is based not only on their raw skill and potential, but is highly influenced by the perceived value of the position they play as well as the overall supply of players at that position. While quarterback is clearly the most important position on the team, investigating where players at other positions get drafted may provide insights into how NFL teams evaluate the relative importance of those positions. In this post I do a couple of simple analyses of where players get drafted, breaking the data down by position groupings and finding that there are slight variations in where players at different positions can expect to be taken, although more draft data and/or deeper analyses would be necessary to decisively show disparities between the positions.

Introduction
It's pretty clear that quarterback is the most important position on a football team, and their value is reflected in the draft – in the last decade 13 QBs have been taken in the top five picks. But exactly how much are QBs favored by GMs and coaches come May? And what about the perceived value of other NFL position groups?

Data
This one's fairly easy, as Armchair Analysis lists where each player was drafted as well as what position they play in the same table. I downsampled the data to include only players drafted since 2001 (up to 2011, the last year in the database), because the table contained only partial records before that year. You can find the script I used here.

There is also a limit to the granularity of the positions in the database. For instance, no distinction is made between any of the players on the offensive or defensive lines. This does limit how detailed I can make my analyses, and there may be significant difference in the valuations between positions on the O-line (for instance, a left tackle–protecting a passer's blind side–is likely to be more highly sought-after than a right tackle, although things are never that clear-cut).

Results
I first took a look at the raw data, plotting what fraction of picks go to each position grouping in Figure 1. To improve the signal-to-noise of the data (and just make things easier to visualize) I binned the data in groups of 10 picks. 
Figure 1: Percentage of players drafted at each position, as a function of draft position. Kickers added for scale.
While colorful, the stacked nature of this figure makes it somewhat difficult to parse. Figure 2 shows where players of each position get drafted, independent of any other position groups in the sample. While there are still a bunch of overlapping lines, it's now easy to see if and where teams prefer to draft players at each position.
Figure 2: Where players in each position grouping get picked. Most of the positions have flat distributions. The only notable exception to this trend are QBs, which are highly peaked near the first few picks.

What stands out most is the large (and unsurprising) upturn in QB picks in the first bin. Otherwise, however, there don't appear to be any obvious trends. But the eye can deceive, so let's try to be just a bit more rigorous. Toward that end I computed the expectation value for each position. You can get more detail about the expectation value from Wikipedia, but in this case it's basically just the average place players at each position get drafted. You can't make a pretty graph with it, but you can see the results in Table 1.

Table 1: Draft position as a function of player position
PositionQBRBWRTEOLDLLBDBK
Expected Draft Position102±8.1126±5.1117±4.4131±5.8122±4.0114±3.9118±4.2119±3.2164±9.1

Table 1 starts off by confirming what we saw visually in the figures – quarterbacks are by far the most sought-after position, with an expected draft position 12 picks higher than any other position (although the bootstrapped standard deviations are just consistent with defensive linemen being equally valued). Wide receivers are drafted slightly earlier than running backs, a trend that's been picking up steam in recent years as teams realize that RBs tend to have short careers and therefore don't provide as much value for an early pick. Linemen are the highest-drafted of all defensive position groupings, probably driven by teams' desire to press the point of attack – the general wisdom is that QBs have an advantage against the secondary due both to their skill and rules restricting contact on WRs (although the Seattle Seahawks would beg to differ) , and that generating pressure and sacks is seen as the best way to defend the pass. More data would be required to definitively prove that these differences are real, however.

Discussion and Conclusions
While it's unsurprising that QBs are the hottest position in the draft, it's nice to see it confirmed with numbers. More interesting are the expectation values for the other positions, which while much closer to each other could be used as signifiers of broad NFL trends in how talent is evaluated between positions. The analyses presented here are pretty simplistic, but they are indicative of the potential power of draft data. 

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