Looking at last season's results, which amount to a data set of 7272 debates, the Affirmative won 52.5% of the time. Whether one considers this a large split is somewhat subjective, but using a binomial distribution we can say that the odds of this happening randomly are functionally zero. In terms of the Glicko ratings, a team gains something like a 17 point advantage by virtue of being affirmative.
To be clear, these numbers do not speak to the cause of the bias. Some might like to say that it's the proliferation of non-topical cases or "new debate." Or perhaps it could be due to the death of case debate, a general favoring of "Aff choice" on framework, or a biased resolution. However, the cause is a question for another day - a question that would require a lot more work than I'm willing to give it at the moment. What I can say is that it cannot be explained by the possibility that stronger teams somehow managed to be Aff more often. I don't know how this would even happen in the first place, but it's not borne out by the data anyway. The average Aff rating is only a fraction of a point higher than the average Neg rating.
While it is possible that the bias was due to the nature of last year's resolution, so far the trend has continued this year as well. Aff win percentage is 52.2% over 2248 counted rounds, which would only happen randomly about 2% of the time. Perhaps the advantage will level out as the season goes on. Conventional wisdom sometimes says that the Aff's advantage is greatest at the beginning of the season when teams have not yet had the chance to fully prepare negative strategies. However, if last season is any indication, we cannot expect to see any evening out. Aff win percentage fluctuated a bit over the course of the year, but it always stayed in the black. In fact, records showed the greatest advantage for the Aff during the second semester period in between the swing tournaments and district qualifiers.
The effect may also apply more narrowly to elimination rounds, perhaps even being magnified depending on how one interprets the data. Keep in mind that the size of the sample is much smaller. Whereas the total data set for 2014-15 was 7272 debates, there are only 563 elimination rounds counted. Of these, the affirmative won 54.4% - a definite step up from the prelim data. However, it's also possible to interpret the results in a way that suggests that the teams that were on the Aff were overall slightly better than the negatives. While the mean rating for negatives was about 5 points higher than for the affirmative, it happens to be the case that in 52.1% of debates the Aff was favored to win. In the end, the bias in elims appears to have been probably about the same as prelims. However, the smaller sample size means that randomness is more likely to have played a role. We could expect to see numbers like these or worse around 14% of the time even if sides were equally balanced.
The data for this year is even more limited. I have 184 elim rounds, of which the affirmative has won 57.6%, a number that is quite high. However, once again, the Aff was also the favored team in 53.9% of these debates, suggesting that the advantage might not be that different than it has been for prelims.
The graph below breaks down the 2014-15 season to more narrowly show how each side performed as varying degrees of favorites. The breakdown suggests a larger bias for Affs at lower point spreads that evens out as the gap between teams rises. The data does provide some support for Rashad Evans's hypothesis that bigger upsets can be scored as the negative. Once the point spread reaches around 300, Neg win percentage starts to outpace the Aff -- though, of course, at this point any chance of upset is very small.