Coverage is measured by total clips plus total stories from 538’s media tracking data. The exclusion of low polling weeks is quite significant for a handful of low-polling candidates, but is necessary to adequately show the relationship between the major candidates.

If we look more carefully at the numbers over time, they usually correlate both within any given week and any given month. “Horse race” coverage is widespread, and most candidates get similar levels of coverage per point of polling most of the time.

Some of the individual exceptions in a particular week or month can be explained by news events. To avoid concentrating on the effect of any singular news event, I’ll be looking at the relationship between coverage and polling in a median, or typical, week. When the same candidates are repeatedly and consistently over- or under- covered relative to their polling, week after week, media bias is the simplest explanation.

Median coverage per polling point; median difference between actual and predicted amount of coverage using a linear model; and median polling average.

The relationship between polling and coverage is not exactly proportionate; it tends to deviate at the higher and lower ends. The natural answer to this question is to do a linear regression.

This shows us two slightly different but related measures of of media bias. The linear model is preferable as it takes into account more information and doesn’t involve dividing by numbers that approach zero.

Looking at both measures, it’s clear that Beto O’Rourke has been shortchanged, with the second-lowest ratio of all candidates in spite of averaging only seventh in the polls. We can similarly highlight Andrew Yang, Steve Bullock, Joe Sestak, and Mike Gravel as routinely under-covered relative to their polling.

On the side of positive coverage bias, the media has preferred to cover Elizabeth Warren, Cory Booker, Kirsten Gillibrand, and Bill de Blasio. Perhaps remarkably, Pete Buttigieg seems to be covered exactly the appropriate amount using either measure. For all other candidates, the indicators are more mixed; we can make a few relative determinations between the other candidates, but it’s not clear they experience either net positive or net negative bias in their volume of media coverage.

Candidates as competitors for coverage: Evaluating every case

There are a couple of major issues we’ve touched on here. One is that polling and coverage have a mostly — but not entirely — proportionate relationship. The anomalies we can identify confidently are based on pairwise comparisons between candidates who have relatively similar polling numbers. Fortunately, we can take all of those anomalous pairwise comparisons and use them to generate a rating of the direction of media bias relative to polling.

As of my analysis, there are 921 cases in the data where a candidate received less coverage than another candidate over the course of a week, in spite of the fact that they were polling better both during that week and the week before. There’s a great mathematical tool for teasing out the relative strength of competitors from a sequence of pairwise competitions: It’s called an Elo rating, after physicist and chess player Arpad Elo.

Here, the polling average is determined by aggregating over all polls in the frame, so the figures are slightly different.

Running the 921 pairwise comparisons through the Elo rating formula gives us a measure of unexplained media preference for (or against) a candidate.

There are two candidates whose ratings indicate they are very strongly snubbed by the media: Andrew Yang (rated 675) and Beto O’Rourke (rated 836).

It’s not clear if third place properly belongs to Bernie Sanders (1022) or Joe Biden (958). Since Joe Biden has been the polling front-runner every single week of the race, his rating is artificially depressed by this measure; he can only “lose” to candidates with lower polling numbers. Moderate politicians as a group tend to be below average. Surprisingly, this includes Pete Buttigieg (1068), who looks close to average in other views of the subject.

The notable candidates with high ratings include Kirsten Gillibrand (1408) and Elizabeth Warren (1335); of the candidates still in the race, Elizabeth Warren is a clear media favorite. The top rated candidates by this method are Eric Swalwell, Bill de Blasio, Jay Inslee, and Seth Moulton. It’s worth asking — again — what the scale of importance is?

How many stories and clips are these candidates gaining and losing?

If we take the 921 pairwise comparisons in the data set and pare them down to each week, we can identify a minimum shift in coverage that would eliminate any apparent snubs. Doing this puts the positive volume bias into perspective: Seth Moulton may have been covered instead of other obscure candidates, but very little coverage was at stake.

Cory Booker stands out by this measure, surpassing the usual suspects of Kirsten Gillibrand and Elizabeth Warren. The most snubbed candidates have again clearly been Beto O’Rourke and Andrew Yang.

A small note on the geography of the volume bias in the media

Sorting candidates by the geography of their constituents (coastal mainland, inland mainland, non-contiguous region, or lacking previous constituents) shows a very interesting pattern.

The ten candidates whose previous constituencies were coastal states, districts, or cities have been covered more. Inland candidates have been covered less. This is actually a remarkably consistent division; it’s also understandable given the geography and demography of the major media outlets.

There are no inland candidates who have unambiguously positive volume bias; the closest are Pete Buttigieg and Amy Klobuchar, who are close to neutral by most measures. There are no coastal candidates with unambiguously negative volume bias; the closest is Joe Biden, who nevertheless leads the coverage race in absolute terms.

Since the political battlegrounds of the Electoral College are neither located in the Acela Corridor nor the West Coast, a geographic bias in the media may impede the nomination of a more electable candidate.

Impacts and effects

From the analysis above, there are a few clear conclusions. First, we can clearly identify a handful of candidates who seem to have attracted a disproportionate amount of media coverage. Elizabeth Warren, Cory Booker, Kirsten Gillibrand, and Bill de Blasio all feature prominently in media coverage compared to candidates who had similar polling numbers. Since Elizabeth Warren’s coverage in “blue” media has also been consistently positive in tone, this has likely fueled her rise.

Second, we can identify two candidates who have very clearly affected by media bias. Both Andrew Yang and Beto O’Rourke have experienced disproportionately sparse and negative media coverage.. It is no wonder that Beto struggled and chose to drop out, or that Andrew Yang’s supporters are griping about the #YangMediaBlackout.

Based on his fundraising and polling, Andrew Yang should have been a larger part of the public conversation earlier. The consequences of failing to do so are negative.

Third, there are a number of lower-profile candidates who were starved for coverage and haven’t been able to get enough air time to make their case to prospective voters. This includes John Hickenlooper and Tim Ryan, who have dropped out; as well as Michael Bennet, Steve Bullock, John Delaney, and Joe Sestak. All of these candidates have backgrounds that are more typical for presidential candidates than Pete Buttigieg.

Sanders’ polling is steady; Gabbard’s polling has been climbing in spite of (or perhaps because of) a recent spate of negative coverage.

Fourth, there’s some evidence for bias in volume to support the frequent accusations of bias in tone made by supporters of Bernie Sanders and Tulsi Gabbard. Neither of those candidates, nor Andrew Yang, appear likely to drop out.

The easily demonstrable media bias, if it continues, is likely to cause harm to the eventual Democratic nominee. The DNC may face another round of accusations about trying to rig the primary contest. Media outlets themselves also stand to lose credibility by engaging in blatant bias, either in volume or in tone.

Given that accusations of bias both by the DNC and by “blue” media have been credible and warranted, it’s all the more important to push for fair, transparent, and impartial coverage of the candidates going into the primary.