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Last Updated: Tuesday, 19 April 2005, 09:30 GMT 10:30 UK
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How the seat calculator works
The BBC News election website seat calculator is a tool that takes vote share percentages for the main parties, and others, and turns them into a predicted election result.

It is possible to enter any figures, but realistic ones are best and a good place to start is with the voting intention figures from recent opinion polls.

But there are a number of important factors worth bearing in mind while using the calculator.

THE METHOD USED

The calculator does not work by simply allocating seats in the House of Commons in proportion to the vote share percentage entered.

For example, if you enter 25% for every party and others, the total number of Commons seats will not be divided up so they all get a quarter each.

 OTHER SEAT CALCULATORS The BBC News election website is not the only place you can use a seat calculator Other organisations and individuals offer alternatives You can see examples using the links on the right

This is because the British first-past-the-post electoral system does not work like this, and the calculator is designed to be as realistic as possible.

For the same reason, it is designed to reflect the current political situation so that the predictions it gives are relevant to the current general election campaign.

To do this, a key part of each calculation involves results from the last general election, in 2001.

The only exception to this is for seats in Scotland, where there are changes to the boundaries of many seats so we have used "notional" 2001 results.

The notional results have been worked out by experts, who looked at the new boundaries and decided what the result would have been had they been in force in 2001.

Proportional shift

A simple way for the seat calculator to work - and the method we used for the same tool in the 2001 election - uses what is known as "uniform swing".

This means if the figures entered give a party a 5% greater share of the vote than they received in the last election, the calculator assumes the party's vote has gone up by 5% in every seat (and the same applies for decreases).

Another way of performing the calculation is to increase or decrease votes proportionally - if a party goes from 30% to 33%, their vote in each constituency would rise by 10%.

 VOTE SHARE MOVEMENT You don't have to enter vote share percentages - you can also input changes from 2001 To do this, put a + or - symbol in front of a figure So, +5 will give a party 5% more votes than in 2001, -5 will give it 5% fewer It's best if any movement is balanced, so if you enter +5 for one party, enter -5 for another
Yet another way is to treat losing and gaining parties differently. Parties whose vote share goes down have their vote reduced proportionally. But the votes they lose are then redistributed, on a seat by seat basis, between the parties that have gained vote share.

All the different methods have their disadvantages, but we have chosen the latter option as we think it gives the most accurate results.

Having said that, no seat calculator will ever give a 100% accurate prediction.

For example, local factors - such as an independent campaign with a good chance of success - cannot be taken into account.

FINE TUNING

The "fine tune" option on the calculator allows the impact of tactical voting to be taken into account.

Tactical voting is a phenomenon where voters cast their ballots not because they support a particular candidate, but because they want to prevent another from winning.

The impact is seen most obviously in marginal constituencies, where two parties face a particularly close fight.

The seat calculator uses quite a strict definition of what a marginal seat is (the winning majority in 2001 has to be less than 10%), and the fine tuning only affects those seats, so any changes to parties' overall seat totals may be small.

Marginal battleground

The fine tuning works by, for example, entering +5 to boost a party's vote share by 5% in the relevant marginal battleground. Entering -5 takes the same amount of seats away.

Where one party gains extra votes, another party must lose votes. So, to work properly, the figures should sum to zero within each marginal battleground column. If one party gains 5% vote share, another party or parties must lose 5%. If the figures inputted do not sum to zero, the calculator rescales them automatically so they do.

Like the rest of the calculator, fine tuning also works best with realistic figures. So, although tactical voting is statistically hard to analyse, our experts suggest using only single figures (in the range of +1 to +9, or -1 to -9) in the table.

GEOGRAPHICAL VARIATIONS

Another option on the calculator is to see predictions for parties standing in Scotland, Wales and Northern Ireland.

The calculation is the same as that used on the UK-wide option, although the predicted results are only for the seats within each geographical area rather than the House of Commons as a whole.

It is not possible to fine tune the Scotland, Wales and Northern Ireland options with tactical voting.

DON'T FORGET

There are other important things to remember when using the calculator:

• It is best to fill in all the vote share percentage boxes, but not necessary. Any box left blank will leave the party's vote percentage unchanged from the 2001 position

• However, if the figures entered do not add up to 100%, they will be rescaled automatically

• You can enter a zero percentage vote share for any party, or 100%, but the calculator only works properly with realistic figures and it is highly unlikely that any party listed on the calculator - or others - will get 0% or 100% of the vote.

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