Let us start by reviewing some recent elections:
| Election | Previous election | Prev LibDem Vote | New LibDem Vote | Prev Seats | New Seats | Vote Change | Seat Change |
|---|---|---|---|---|---|---|---|
| 1997 | 1992 (implied seats) | 18.3% | 17.2% | 18 | 46 | -1.2% | 28 |
| 2001 | 1997 | 17.2% | 18.8% | 46 | 52 | +1.6% | 6 |
| 2005 | 2001 (implied seats) | 18.8% | 22.7% | 51 | 62 | +3.8% | 11 |
Let us have a look at these three elections in detail, including seeing them seat-by-seat.
We can compare this outcome with what different uniform-swing models predict. The standard additive model predicts that all the yellow dots would lie along a straight line with slope 1.00 (with the line passing through the point (18.3%, 17.2%) which marks the average LibDem support in 1992 and 1997 respectively). The Electoral Calculus Transition Model predicts a straight line with slope 0.94 (=17.2%/18.3%), passing through the same average point. The additive model is closer to the actual outcome, but both models do not predict the cluster of seats around 40% which lie above the line. These seats, which are mostly won by the Lib Dems, are seats where the Lib Dems have done much better than any uniform swing predicts. It is behaviour like this which makes predicting the Lib Dems difficult.
This seat information can also be seen in a table. The table divides seats up into groups, depending on the Lib Dem share of the vote in each seat in 1992. The first group is those seats where the Lib Dems received between 0% and 10% of the vote, the second group goes from 10% to 20%, and so on up to the strongest seats where their support is between 60% and 70%. For each group we can measure the average support level in 1992 (which is around the mid-point of the group's range), and also the average support level for those same seats in 1997. This shows how support changes, depending on the starting support level. Also shown are the number of seats won by the Lib Dems in each category.
| Low | High | Count | 1992 Ave | 1997 Ave | Change | 1992 Wins | 1997 Wins |
|---|---|---|---|---|---|---|---|
| 0% | 10% | 129 | 8.0% | 7.9% | 0.0% | 0 | 0 |
| 10% | 20% | 300 | 13.9% | 11.7% | -2.2% | 0 | 0 |
| 20% | 30% | 129 | 24.9% | 23.1% | -1.8% | 1 | 4 |
| 30% | 40% | 60 | 34.4% | 34.6% | +0.2% | 4 | 20 |
| 40% | 50% | 20 | 44.2% | 45.9% | +1.7% | 10 | 19 |
| 50% | 60% | 3 | 51.2% | 48.6% | -2.6% | 3 | 3 |
| 60% | 70% | 0 | 0.0% | 0.0% | 0.0% | 0 | 0 |
| 641 | -1.3% | 18 | 46 |
Again we can compare the actual movements with uniform swing predictions. Under the additive model, we should expect that the change will be equal for each group at around -1.2%. Under the transition model, we expect that changes in the weaker seats will be smaller (-0.5% for seats in the 0%-10% range), and greater for stronger seats (-3.1% for seats in the 50%-60% range). Broadly speaking the groups for 0%-10%, 20%-30% and 50%-60% behave more like the transition model and the additive model. However, the groups for 10%-20%, 30%-40% and 40%-50% behave like neither. The first group, which contains the bulk of the seats, performs worse than expected under either model. The other two groups actually gain support, against the national trend, and gain quite a number of seats.
This reflects very well on the Liberal Democrats' electoral strategy which is able to "lose" votes where it does not matter (both in seats where they are weak or very strong), and to "win" votes in target marginal seats. But it does make them hard to predict. The final Electoral Calculus prediction in 1997 for the Lib Dems was too low by 21 seats.
This election was a little easier to predict. The points are fairly evenly distributed along the best-fit line, apart from a couple of outliers. The table breaking down seats according to their 1997 vote share shows a similar picture. Some groups (10%-20%, 30%-40%, 40%-50%) move similarly to the national +1.5% change. The exceptions this year are in groups which do not change the seat winner, such as the weakest and strongest groups along with the 20%-30% group.
| Low | High | Count | 1997 Ave | 2001 Ave | Change | 1997 Wins | 2001 Wins |
|---|---|---|---|---|---|---|---|
| 0% | 10% | 196 | 7.8% | 10.5% | 2.7% | 0 | 0 |
| 10% | 20% | 283 | 13.4% | 14.7% | 1.3% | 0 | 0 |
| 20% | 30% | 80 | 25.2% | 24.7% | -0.5% | 0 | 2 |
| 30% | 40% | 45 | 35.3% | 37.4% | 2.1% | 10 | 16 |
| 40% | 50% | 29 | 44.5% | 45.5% | 1.0% | 28 | 26 |
| 50% | 60% | 8 | 52.2% | 50.1% | -2.1% | 8 | 8 |
| 60% | 70% | 0 | 0.0% | 0.0% | 0.0% | 0 | 0 |
| 641 | +1.5% | 46 | 52 |
The error in the Electoral Calculus prediction for the Liberal Democrats in 2001 was only 2 seats. See the 2001 track record.
The best-fit line is relatively good, but there is a small but noticeable set of seats around 25% support in 2001 which lie well above it. The table breakdown shows this clearly as the unexpected gains of seats in the 20%-30% range from 0 seats to 10 seats. The bulk of seats in the ranges from 0% to 30% swing as expected at around +4%. But the strong seats from 30% to 50% gain less, and the strongest seats lose support. This meant it was hard to predict the wins in the 20%-30% range, and easy to overstate the expected gains in the 30%-40% group.
| Low | High | Count | 2001 Ave | 2005 Ave | Change | 2001 Wins | 2005 Wins |
|---|---|---|---|---|---|---|---|
| 0% | 10% | 89 | 8.1% | 12.9% | 4.8% | 0 | 0 |
| 10% | 20% | 371 | 13.8% | 18.3% | 4.5% | 0 | 1 |
| 20% | 30% | 85 | 24.4% | 28.8% | 4.3% | 0 | 10 |
| 30% | 40% | 33 | 35.0% | 36.7% | 1.6% | 6 | 8 |
| 40% | 50% | 37 | 44.7% | 45.7% | 1.0% | 32 | 30 |
| 50% | 60% | 12 | 53.3% | 49.2% | -4.1% | 12 | 12 |
| 60% | 70% | 1 | 60.2% | 51.0% | -9.1% | 1 | 1 |
| 628 | 4.0% | 51 | 62 |
The error in the Electoral Calculus prediction for the Liberal Democrats in 2005 was only 4 seats (too low), though this was made up of eight seats down and four seats up. See the 2005 track record.
The Liberals have never won fewer than six seats in any election from 1900 onwards.
We can note that the difference between the transition model and the additive model for the Lib Dems is only about 15 seats. Whilst this is a very important difference for Liberal Democrat MPs and supporters, it is not very material in calculating which of the major parties, if either, will have a majority in the House of Commons. More important is whether the Lib Dems will do much better than either uniform-swing model suggests and manage to hold 30 or more seats and have the potential to be power-brokers in a hung parliament.