Safe Seats: Scenario Analysis

This page first posted 13 September 2009, updated 6 March 2010, 24 April 2010

This page is part of an analysis about which seats are safe.

1. Poll Analysis

We start by looking at all the published national opinion polls from May 2005 to date (6 Sep 2009). There are 370 relevant polls. To define the edges of our extreme scenarios we look at various percentiles of the distribution of party support. For instance the Conservatives support level at the 25% percentile is 36%. This means that in one-quarter (25%) of those polls the Conservatives had support less than 36%, and in three-quarters of the polls they had support more than 36%. The central 50% percentile is also called the median.

The table shows some key percentiles of party support:

Percentile	5%	10%	25%	50%	75%	90%	95%
Con %	32	33	36	39	41	44	45
Lab %	24	25	29	32	36	39	40
Lib %	14	14	16	18	19	21	22
All Others %	7	8	9	11	13	15	18

What does this table tell us? Looking at the Conservatives first, it shows that their central support level is 39%, which happens to be close to their current support level of 40%. It also shows that half of all the polls have them in the range from 36% to 41% (from the 25% to 75% percentiles). And, in the extreme case, nine out of ten polls have them in the range from 32% to 45% (from the 5% to 95% percentiles). We will use this extreme case as the basis for our scenarios of Conservative support. So we assume that 32% is the worst reasonably possible result for the Conservatives and that 45% is the best reasonably possible result.

For Labour, the central support level is 32% and the range of worst to best outcomes goes from 24% to 40%.

For Liberal Democrats, the central level is 18% and their range goes from 14% to 22%. Because Lib Dem support often increases during the election campaign, we will extend their best-case range to 25% to make sure that all (reasonably) possible outcomes are included.

We could instead have just used the simple minimum and maximum of poll support levels. Why did we use this more complicated method instead? The answer is that the minimum and maximum can be skewed by a single rogue poll. Since we are using several hundred polls, the chance of at least one or two bad ones is quite high. We can reduce the risk of such "outliers" by using a percentile range instead of the full range. Almost all polls are included, but we ignore a small number of polls (37) that are very far from all the others.

2. Scenario Definition

Now we have our ranges of plausible party support, we can create some scenarios. We want to have a number of scenarios, each of which describes the support of the three major parties. The aim is to have the complete set of scenarios span the extremes of possible voter behaviour.

We have used six scenarios. Three scenarios correspond to one major party doing well, and the other two doing badly. The other three scenarios correspond to two major parties doing well, and the other one doing badly. The scenarios are not meant to illustrate likely outcomes, but rather to describe the limits of credible outcomes. It is likely that the eventual outcome will be within the limits spanned by the scenarios.

For information the base scenario (general election May 2005) is also shown. The scenarios are:

Scenario	Strong	Weak	Con %	Lab %	Lib %	Swing	Outcome	Con seats	Lab seats	Lib seats	Nat seats	ChangedSeats
May 2005			33	36	23	0	Lab maj 42	208	346	67	8	0
One	Lab	Con, Lib	32	40	14	-2.5	Lab maj 118	198	384	35	12	60
Two	Con	Lab, Lib	45	24	14	12	Con maj 200	425	154	22	28	238
Three	Lib	Con, Lab	32	24	25	5.5	Con short -41	284	238	89	18	111
Four	Lab, Lib	Con	31	40	25	-2.5	Lab maj 110	173	380	69	7	39
Five	Con, Lib	Lab	45	24	25	12	Con maj 166	408	148	65	9	217
Six	Con, Lab	Lib	44	39	13	4	Con short -18	307	306	10	6	111

(Note: some scenarios have adjusted the ranges slightly to make sure that the total of support, including nationalists, is below 100%.)

The scenarios include a range of possible outcomes

Two cases where Labour keep power and increase their majority
Two cases where the Conservatives get a large majority
Two hung parliaments, including one where any government would need Lib Dem support

We note that over all these extreme scenarios, the largest number of seats which ever change hands is 238 (less than 40% of the total number of seats). So under each individual scenario, six out of ten seats are safe.

3. Scenarios in Scotland

As part of this analysis we also looked at Scotland separately. Since there are far fewer opinion polls, we used a simple minimum and maximum to get the likely range of support.

	Min	Median	Max
Con %	14	18	20
Lab %	28	36	44
Lib %	9	14	23
SNP %	16	30	34

Again we adjust the maximum of the Lib Dems range up to 25% since no opinion poll has shown them increase their support since the general election. Under each of the six GB scenarios above, we specify the particular share of the vote that the four Scottish parties receive. We assume that the SNP does well in the first three GB scenarios, and does badly in the other three GB scenarios.

Scenario	Strong	Weak	Scot Con %	Scot Lab %	Scot Lib %	Scot Nat %	Con seats	Lab seats	Lib seats	SNP seats	Changes
May 2005			16	40	23	18	1	41	11	6	0
One	Lab, SNP	Con, Lib	14	43	9	33	1	43	6	9	7
Two	Con, SNP	Lab, Lib	20	28	9	34	8	22	6	23	24
Three	Lib, SNP	Con, Lab	14	28	24	33	2	28	15	14	13
Four	Lab, Lib	Con, SNP	14	44	25	16	0	43	11	5	2
Five	Con, Lib	Lab, SNP	20	28	25	16	6	33	15	5	10
Six	Con, Lab	Lib, SNP	20	44	9	16	6	44	6	3	8

We see that the scenarios generate a range of outcomes. There is the possibility for any party to increase or decrease its number of seats.

4. Special cases

At the 2005 general election, three seats were won by independent or minor party candidates. Since these seats are hard to predict scientifically, we will assume that they are not "safe" and could change hands. We also include otherwise safe seats which have changed hands at by-elections. These seats are:

Bethnal Green and Bow (Tower Hamlets/North London) - George Galloway MP, Respect the Unity Coalition
Blaenau Gwent (Mid Glamorgan and Gwent/Wales) - Peter Law MP, Independent; succeeded by Dai Davies MP, Independent.
Glasgow East (Glasgow area/West Scotland) - John Mason MP, SNP (by-election gain)
Wyre Forest (Hereford and Worcestershire/Severn) - Dr Richard Taylor MP, Kidderminster Hospital and Health Concern

5. Total numbers of "safe" and "unsafe" seats

We can define a seat to be "unsafe" if any of these conditions apply

The seat would change hands under any one or more of the six scenarios, or
The seat is held by an independent or by-election victor, as listed in section four.

There are 306 "unsafe" seats, out of a GB total of 632 seats.

Conversely a seat is defined to be "safe" if none of the conditions above hold. In other words, a seat is safe if it never changes hands under any plausible scenario of public opinion. There are 326 "safe" seats, which is about half of the total.

Caveat: Of course, nothing is entirely 100% safe. It is possible that extreme public opinion swings or strong local factors in a particular seat may make a "safe" seat change hands. But within the bounds of the likely set of outcomes and the law of averages, it is pretty unlikely that more than a handful of safe seats could be captured.

6. Campaign update - 24 April 2010

During the election campaign there was a significant increase in Lib Dem support up to 34% in opinion polls. This caused some seats which were deemed "safe" to be predicted Lib Dem gains. To cope with this unexpected development, we have redefined scenarios Three, Four and Five which have strong Lib Dem performance. These scenarios are now:

Scenario	Strong	Weak	Con %	Lab %	Lib %	Swing	Outcome	Con seats	Lab seats	Lib seats	Nat seats	ChangedSeats
Three	Lib	Con, Lab	32	24	33	5.5	Con short -65	260	207	144	18	154
Four	Lab, Lib	Con	29	38	31	-3.0	Lab maj 92	160	371	91	7	54
Five	Con, Lib	Lab	43	22	31	12	Con maj 126	388	135	98	9	221

There are now 300 safe seats, and 332 unsafe seats in Great Britain.