Since the runoff McDaniel and his supporters have complained that McDaniel actually won the Republican vote in the runoff and Cochran was only the winner through getting African Americans and Democrats to vote for him.
As The New York Times' Upshot blog notes, Cochran won 92 percent of the votes in the precincts that are Democratic strongholds. President Barack Obama won 99 percent of the vote in those precincts in 2012. Cochran won those precincts by 3,889 votes, or more than half of his margin of victory in the runoff over all. With those votes, Democratic votes in other parts of the state could have cleanly brought Cochran over the finish line in the runoff, even with Republican voters in those precincts going for McDaniel.
An important data point to keep in mind also is that in historically Democratic-dominant places like Hinds County, one of the most densely populated counties in Mississippi, turnout wasn't especially large (7.4 percent of the 2012 general election numbers) — but with runoff voters going 92 to 8 for Cochran, it would be enough to move the race in Cochran's favor.
Cochran also got about 32,000 votes from blacks who voted for Obama in the last election, according to the Upshot. That's about 8 percent of the 382,000 votes over all cast in the runoff election.
Mississippi is a fairly racially polarized state so, even though how African-Americans voted in the 2012 presidential election isn't perfectly representative of how African-Americans voted since then, it's fairly close, according to The Upshot. According to exit polls, only about 10 percent of the state's white voters went for Obama in 2012.
In the whitest, most Republican-leaning precincts (places where Obama got less than 20 percent of the vote) McDaniel beat Cochran with 19,033 votes or 57 percent of the vote. Again though, those numbers don't perfectly represent all white voters.
Another important fact to keep in mind: the new data doesn't confirm or disprove that claims by McDaniel supporters that Cochran got Democrats who voted in the Democratic primary to illegally vote in the Republican primary.
The new data is based on 93 percent of the runoff electorate. The other 7 percent is based on information from precincts that couldn't easily be compared to the 2012 presidential election. But, as the Upshot notes, it isn't likely that the remaining unmatched data would change the other findings. The unmatched data came from precincts that leaned more Democratic, meaning they were likely more favorable to Cochran.