Extensive AP analysis using the efficiency gap:

The AP scrutinized the outcomes of all 435 U.S. House races and about 4,700 state House and Assembly seats up for election last year using a new statistical method of calculating partisan advantage. It’s designed to detect cases in which one party may have won, widened or retained its grip on power through political gerrymandering.

The analysis found four times as many states with Republican-skewed state House or Assembly districts than Democratic ones. Among the two dozen most populated states that determine the vast majority of Congress, there were nearly three times as many with Republican-tilted U.S. House districts.

Traditional battlegrounds such as Michigan, North Carolina, Pennsylvania, Wisconsin, Florida and Virginia were among those with significant Republican advantages in their U.S. or state House races. All had districts drawn by Republicans after the last Census in 2010.

The AP analysis also found that Republicans won as many as 22 additional U.S. House seats over what would have been expected based on the average vote share in congressional districts across the country. That helped provide the GOP with a comfortable majority over Democrats instead of a narrow one.