BREAKING: A statistical pet peeve!


Statewide congressional vote versus gerrymandering:
Gerrymandering can sometimes be in the eye of the beholder.

That pretty much isn't the case with Pennsylvania's current congressional districts. The 18 districts are so wildly irregular that they would look crazily gerrymandered to pretty much anyone's eye.

For one crazy-quilt example, see this Kevin Drum post. On the down side, Drum advances one of our favorite "pet peeve" statistical howlers at the start of that very same post:
DRUM (1/23/18): Yesterday the Pennsylvania Supreme Court declared the state’s congressional districts unconstitutional because they had been so badly gerrymandered. The result of the 2016 election bears out how effective the gerrymander was: Republicans won 54 percent of the congressional vote but received 72 percent of the congressional seats (13 out of 18).
Doggone it! That familiar construction suggests that a party which gets 54 percent of the statewide House vote should get something resembling 54 percent of the congressional seats. It certainly can work out that way, but it ain't necessarily so.

Imagine a state with 18 non-gerrymandered House districts. The imaginary state's congressional districts are as "regular" as can be.

This state's imaginary electorate is 54 percent Republican, 46 percent Democratic. But also imagine this:

That population is evenly spread across the state's 18 districts. In each of the 18 congressional districts, the electorate is 54 percent Republican.

In a state like that, the GOP would get 54 percent of the statewide vote, and it would win every single congressional seat! In principle, no gerrymandering is required to produce an outcome like that.

In the real world, Pennsylvania's districts plainly have been gerrymandered. No one seems to dispute this. But every time you see a gap between the statewide vote and the statewide allotment of districts, you aren't necessarily seeing evidence of foul play.

As has been widely observed, gerrymandering isn't the only force which can tilt the allotment of House against the Democratic (or Republican) party. As has been widely observed, Democratic voters tend to be heavily concentrated in large urban areas. In the absence of gerrymandering, this tends to produce urban House districts which are heavily Democratic.

In states which feature such concentrations, Democrats may win a smaller number of districts by huge margins. Republicans will thereby win a larger number of districts by smaller margins.

(Our own sprawling campus sits in one such urban district with lots of extra Democratic votes.)

These remarks have nothing to do with the claim that Pennsylvania is gerrymandered. They have to do with a statistical pet peeve, one which drives us wild.

Pictures from the leading authority: How screwy are the shapes of some Pennsylvania districts? The leading authority on the state shows you all the shapes here.


  1. I wonder why redistricting occurs the way it does. Okay, I don’t, I know it’s largely a partisan endeavor. Seems to me the solution would be quite easy. Just give each Representative a boundary within which to operate (a circle would seem logical), which would include Congressional, State Senate, Assembly, City council and School. Then, allow those boundaries to overlap to the point where all Representatives have their required constituents. Voilá! Maybe we’d have a more parliamentary system in each state as a result, and that would be BAD. Problem is, I don’t know where the “center” of each a district should be placed. Maybe centered around school districts?

    I know, I know, I’m not called Simple Simon for nothing.


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  2. Bob Somerby wrote:

    "These remarks have nothing to do with the claim that Pennsylvania is gerrymandered. They have to do with a statistical pet peeve, one which drives us wild."

    Somerby, of course, posits himself as an educational statistics expert. Yet he continuously ignores the fact that his regularly cited statistical "rule of thumb" for NAEP scores, "10 points is roughly equivalent to 1 academic year" is used by nobody except him.

    1. The commenters here are as useless as anywhere else on the Internet, including crazy sites like Breitbart. I don't know why that is.

      In any event, Anonymous, if you simply Google "NAEP scores 10 points rule of thumb," you'll find many sites that "use" that very rule that you claim no one uses except Somerby.

    2. Are you including yourself as one of those useless commenters? Amidst the hateful, troll-like bs comments posted here, you choose to get fired up over this comment, because it criticizes Somerby.

  3. Districts are not supposed to be drawn for political reasons but if the majority party does not get the majority of seats they complain. Being that Democrats congregate into cities those cities have to be sliced like a pizza or tetras shapes to include Republican suburbs.

  4. Gerrymandering to give an unfair advantage to Republicans sounds good to me.

  5. I'm pretty sure Kevin Drum is well-aware of American electoral system being (unfortunately) the first-past-the-post variety, winner takes all.

    The issue is, of course, that Kevin Drum is a repulsive liberal hack who will spout any bullshit for a few 'likes', or, rather, for a few bucks.

    Oh well, all too common these days...

  6. Per usual Dems have to change the rules of the game every time they lose.

    But they should be careful what they wish for. PA's "horrible gerrymandering" is actually quite a risk for Republicans. Many of the districts are R+10 or less. If there's a large Democrat wave election this year, Dems could win every single seat! Even in a mild wave, R's would lose everything except 3-4 seats.

    With districts Dems claim they want, such a wave would be impossible.

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