With the latest batch of Senate polling results, Neil Stevens of Likely voter has updated his Senate Projection. According to his latest model, he projects **R+7**, with a **3%** chance of Republicans reclaiming the majority (R+10 or greater).

The problem I have with the Likely Voter projection model is two-fold: one, it factors in races that are, for all intents and purposes, already decided; and two, it seems to assume a normal distribution. If a projection were performed in which seemingly non-competitive races were removed, and the ANOVA based solely on the actually competitive races, intuition tells me that such a projection (especially with a right-leaning distribution) would have to center around R+8, if not +9. To wit:

## Introducing Uncertainty Based on Non-competitive Races

### Range of Possible Outcomes

First, let's set the table:

- Seats not up for election: 63 (23R, 40D)
- Seats up for election: 37 (18R, 19D)
- Bondary of Possible Results:
**R-18 to R+19**

Now, let's add in some realistic boundaries to those results.

### Range of Realistic Outcomes

#### Lower Boundary of Realistic Outcomes

- All
**18 R-held seats**are 100% likely R.**(R+0)** - The following D-held seats are 100% likely R:
**AR, IN, ND, PA (R+4)**

It is *logical* to conclude that, at this point, any projection that shows *anything less than R+4* is just not consistent with reality.

#### Upper Boundary of Realistic Outcomes

- The following D-held seats are 100% likely D:
**HI, MD, NY, OR, VT (R+14)**

It is *logical* to conclude that, at this point, any projection that shows *anything more than R+14* is just not consistent with reality.

#### Range of Realistic Outcomes

So, at this point, the range of *realistic* outcomes is **R+4 to R+14**. Anything outside of these numbers should be considered *0% likely*.

### Range of Likely Outcomes

- The following D-held seats are 90% likely R:
**CO, WI (R+6, lower)** - The following D-held seats are 90% likely D:
**DE, NY (s) (R+12, upper)**

So, at this point, the range of *likely *outcomes is **R+6 to R+12**. Anything outside of this range should be considered *unlikely*.

Likely Voter's current projection distribution curve has a mean of R+7, and R+5 - R+8 accounts for 77% of all outcomes (R+7 22.6%, R+6 22%, R+5/R+8 35.4%). If I assume that Likely voter's probability curve is normally distributed, then, IMHO, the mean simply *must* be shifted too far left. There is just *no possible way* that R+5 has *18% probability*. I'd say, at the *absolute upper end*, it has 5-10% probability. Balancing the 90% Likely R pickups against the 90% Likely D holds lowers the probability even further.

So, just using back-of-mental-napkin calculations, I would say:

- <R+5: 0% likely
- R+5: 5% likely
- R+6 - R+12: 90% likely
- R+13: 5% likely
- >R+13: 0% likely

### Analysis of Actually Competitive Races

The eventual outcome will be determined entirely by the results of six races: CA, CT, IL, NV, WA, and WV.

Two or three weeks ago, I would have rated those races as follows:

**Lean-R:**The following D-held seats are**55% likely: IL, WV****Toss-Up:**The following D-held seats are**50% likely: NV, WA****Lean-D:**The following D-held seats are**45% likely: CA, CT**

However, things have shifted a bit; I would now rate these races as follows:

**Lean-Likely-R:**The following D-held seats are**60% likely: WV****Lean-R:**The following D-held seats are**55% likely: IL, NV****Toss-Up:**The following D-held seats are**50% likely: CA, WA****Lean-Likely-D:**The following D-held seats are**40% likely: CT**

As you can see, aside from CT (which, to be honest, I am close to writing off as a potential Republican pick-up), all of the competitive races have shifted in the Republicans' favor. I put together a quick Monte Carlo simulation of my own, and here are the results:

So, my model projects a mean * +9 seat gain* for Republicans, and a

**that Republicans will regain control of the Senate (a gain of +10 or more seats). Results:**

*40.1% chance*- n = 10,000
- μ = 5.2
- σ = 1.4
- max = +13
- min = +5
- +8 - +10 = 73.1%
- 10+ = 40.1%

At first blush, these numbers appear to me to be more realistic, given the current state of the races in play (and not in play).

## Evaluating the Normal Distribution Model

It seems that the Likely Voter projection model is based upon the assumption that the outcomes of competitive races will be normally distributed. I wouldn't expect a normal distribution for these outcomes, even in a "normal" election year - but especially not in a "wave" year.

Just as the outcome distribution of competitive races was biased toward the Democrats in 2006 and 2008, I fully expect the distribution to be biased toward the Republicans in 2010. This bias is due primarily to two factors that are not easily accounted for through pre-election polling: the enthusiasm gap and shifts in party affiliation.

In short, pollsters simply don't have a reliable means of estimating the breakdown of voter turnout, and it is entirely likely that they will tend to err on the side of a conservative estimation of the shift from 2006/2008 to 2010.

In a later post, I will examine some of these factors in each of the six competitive races.

Senate Projection for October 21 – http://www.chipbennett.net/2010/10/21/se… #senate #tcot

@presjpolk my thoughts (and amateur projection) based upon your projection: Senate Projection for October 21 – http://www.chipbennett.net/2010/10/21/se…

@chip_bennett Heh, your monte carlo has a mjuch higher peak than mine does.

@presjpolk as would be expected, eh? Totally different assumptions & significantly different estimated probabilities for competitive races

@chip_bennett It’s just fun to see. 🙂