Tactical Voting Parameters

The user-defined prediction also gives a (simple) model of tactical voting, which lets you control the size and direction of any effect.

The idea of the model is that some voters whose first-choice party is likely to come in third place would rather vote for their second-choice party which is more likely to win.

See the Tactical Voting Model for more details about the model itself and the historical experience.


In seats which the Conservative and Labour parties are quite close, and the Liberal Democrats are likely to be in third place, some Liberal Democrat supporters may vote Labour instead. The fraction of voters which vote tactically in this way is assumed fixed at, say, 2% of all those voting in the seat.

We call this a 2% tactical swing from Lib (to Lab).

Input parameters

In the user-defined prediction entry page, there is an area for entering the tactical swing levels:

Party     Tactical Swing
Conservatives % to LIB
Labour % to LIB
Liberal Democrat  % to LAB

In this example, the user specifies zero tactical swing from Con (to Lib), a tactical swing of 1.5% from Lab (to Lib), and a tactical swing of 2% from Lib (to Lab).

Typical parameter sizes

The value of tactical swing parameters should usually be between -5% and +5%. Observed tactical swings almost always lie within this range and are often smaller.

The most important swing is the last one  Liberal Democrat % to LAB  which describes what Lib Dem supporters might do in seats where the Lib Dems are likely to come a weak third. There are far more of these seats than any other type, so this input matters most. In the last two elections, this number was around +5% in 1997 and +2% in 2001 as Lib Dem supporters voted Labour tactically to defeat Conservative candidates. This swing may continue to be positive, but many people believe it will go negative (such as -2%) meaning that Lib Dem supporters will reduce their tactical voting for Labour. Up to 20 seats can be affected by this input.

The key judgement to make is whether this number is positive (Lib Dems continue to support Labour tactically) or negative (tactical "unwind" as fewer Lib Dems supporters vote Labour tactically). Try various values of this parameter and see how the prediction changes.

The secondary input is  Labour % to LIB  which describes how many Labour supporters, in seats where Labour is in third place, will tactically vote for the Liberal Democrats. This swing was around +4% in 1997 and +1% in 2001. This parameter can influence about half-a-dozen seats.

The first input  Conservative % to LIB  is much less important, and is unlikely to influence more than a couple of seats.

Special cases

The algorithm is flexible and copes with some more complicated cases.
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