Every two years the House elections don’t go the way partisans think they should. Every two years partisans on the losing side place the blame on gerrymandering, the purposeful drawing of district lines to achieve a specific outcome. Every two years there is a new study showing that gerrymandering is not the problem. So here we go again.
Jowie Chen and David Cottrell, of University of Michigan and Dartmouth respectively, published a study (PDF) just before the election last year showing that gerrymandering has almost no effect on the overall makeup of the US House of Representatives. Previous studies have shown a significant or minor one, but Chen and Cottrell easily point out the flaws in most of those, especially in how they create a control group. This new study eliminates those inherent biases by using computer models to create a number of different nonpartisan district maps.
As a baseline, they used the 2008 election results at the precinct level.
We map the votes of these precincts to Florida’s 484,481 Census blocks according to population and then aggregate the votes into a set of 15,640 similarly-populated square polygons so as to produce a geographically-precise spatial grid of the state. These 15,640 “squares” of the grid are then used as the building blocks for the districting simulations.
The computer then chooses precincts at random to begin each district and adds more contiguous precincts until a district-size population is reached.