In my last post I looked at the
propensity for coaches to disproportionately use players drafted highly. Higher
draft picks play slightly more than their underlying ability (i.e., performance
over the next few seasons) would predict. In this post I want to take a quick
look at whether this same effect is visible at the margin between 7th
round draft picks and undrafted free agents.
The underlying assumption here is that 7th round
picks are not all that different from undrafted free agents who get a look from
teams. While I would love to be able to validate that assumption with some
data, we don’t exactly have those populations in our available data. What we
can compare is 7th round picks who have made NFL rosters to
undrafted free agents who have made NFL rosters. As I noted in my previous post
on this topic, this obscures the most likely place for this bias to manifest
itself – decisions on who makes the roster – but can’t really be helped.
Approach
From 1994 through 2010[1]
we have 2,043 undrafted free agents and 540 7th round picks who made
NFL rosters in at least one season. This analysis will compare their playing
time – games started are equal to 1 and games played but not started vary by
position as a proportion of games started – with two factors: whether they were
drafted or not and how well they played over the next 3 seasons. Performance
over the next 3 seasons serves as a proxy for underlying skill. I am using the
square root of that performance because I want to weight the player who has a 3
year line like 1-2-13 close to the player whose line is 9-10-8. I am assuming
that both have a similar level of skill but the 1-2-13 player may have been
blocked from starting or overlooked because he was undrafted.
Results
As with the other analysis, it’s important to note first of
all that the relationship here is not that meaningful (R = 0.36, R^2 = 0.13).
For players who never play another season in the NFL, an undrafted one is
expected to play the equivalent of 2.69 games while a 7th round pick
would be expected to play 3.01. Being drafted alone moves the expectation by
0.32 games (p-value 0.03), more than 10% of the baseline. Compared to
underlying skill, however, being drafted is much less meaningful. For the
hypothetical “1-2-13” player above, underlying skill adds 3.29 games to the
expectation (coefficient is 0.82 per unit, p-value 0.00).
Moving to players in their 2nd season the effect
of being drafted goes away completely (p-value 0.99) while underlying skill
becomes more powerful (coefficient is 1.12 per unit, p-value 0.00).
Based on this analysis I am pretty confident that there is a
weak positive effect of being drafted on playing time for rookies. Given the
way it evaporates in the second season I would not be surprised if it is
strongest early in the first season on a per-game basis. I still believe there
is a larger effect that is hidden by lack of data in terms of roster decisions.
If anyone has any idea how to get at this question, feel free to let me know.
[1]
For this analysis only players with at least 3 subsequent seasons possible
(whether played or not) are eligible