House 2008: How many seats will we gain?

reposted from dailyKos, with changes

This is the first in a series estimating how many House seats we will gain in the 2008 election.  For those who like to cut to the chase, my best guess is a net gain of 6 seats.  There is less than a 5% chance that we lose seats, and about a 7% chance that we gain more than 10.

Details below the fold

This is all very preliminary and sketchy.  Part of the reason for posting is to get more info from people who know the individual races. 

Thanks to all the other kossacks who provide information on these races, including Benawu, Adam T, Steve Singiser, superribbie, and Nathaniel Ament Stone (apologies in advance if I am missing someone!)

Here’s what I did.  I went through the wonderful Race Tracker Wiki and looked for any races with a confirmed challenger.  Then I considered a) The partisanship of the district (measured by Bush-Kerry)  b) The personal repuation of the representative (measured by previous race percentage)  c) Whether it was a direct rematch of a previous race  d) Retirement.

I still want to add info on fundraising and, later, polls.

Then I assigned a ‘chance of switching’ to each district.  If there was no reason to suspect a switch, I gave it a .01 chance.  This increased based on all the factors, above.

I simulated 1000 runs of a binomial experiment for each race (using R , added the positive switches, subtracted the negative switches, and voila!

Some problems: 1) A lot of races with no challenger now will have one, although most (but by no means all) of these are races where the challenger will have little chance.  2) I have no idea if my model of percentage chance of switching is correct.  3) It’s early in the season.

But here are the results:

lose seats  4.9%
no gain  3.5
gain 1  5.6
  2    6.5
  3    9.6
  4    11.2
  5    12.5
  6    13.2
  7    9.1
  8    7.1
  9    6.4
  10   2.8
more than 10  6.8 

Below are the seats where I estimate there is more than a 5% chance of switching
Republican held:
AL-03:  7%
AK-AL: 20%, may go up.  Young is in big trouble
AZ-01: 35%, Republican retiring
CA-04: 40%
CA-26:  7%
CA-50:  6%
CA-52: 10%, Republican retiring
CT-04: 40%, possible Republican retirement
DE-AL: 25%
FL-08: 10%
FL-09: 10%
FL-10: 10%, possible Republican retirement
FL-13: 45%
FL-15:  7%
ID-01: 25%
IL-06:  9%
IL-10: 13%
IL-11: 40%
IL-13:  6%
IL-14:  6%, Republican retiring
IL-16:  6%, Republican retiring
IA-04: 10%
MI-07: 14%
MI-09: 14%
MN-03: 30%, possible retirement
MN-06: 10%
NV-03: 30%
NJ-07: 30%
NM-01: 40%
NM-02: 15%
NY-13: 15%
NY-25: 30%
NY-26: 10%
NY-29: 15%
NC-08: 20%
OH-01:  9%
OH-02: 10%
OH-05: 10%, open seat due to death
OH-14:  9%
OH-15: 50%, Republican retiring, Bush-Kerry was even
OH-16:  7%, Republican retiring, Bush-Kerry was 54-46
PA-15: 11%, rematch of close race, Kerry won
PA-18:  9%
VA-11: 40%, possible retirement, Bush-Kerry was even
WA-08: 40%, rematch of close race
WY-AL: 15%

Democratic held
CA-11: 11%
CA-23: 11%
CT-02: 30%
CT-05: 30%
FL-02: 20%
FL-16: 20%
GA-08: 15%
GA-12: 15%
IL-08: 10%
IN-02: 10%
IN-08: 10%
IN-09: 40%
KS-02: 20%
KS-03: 10%
KY-03: 10%
ME-01: 10%, retirement
MN-01: 10%
NH-01: 30%
NY-19: 10%
NY-20: 10%
OH-18: 10%
PA-04: 30%, rematch of close race
PA-10: 10%, rematch of close race
PA-17: 10%
TX-22: 20%, rematch of close race