Bromley and Chislehurst: New Boundaries 2018 Calculation

Bromley and Chislehurst: Overview

See overview of other seats in London.

Introduction

This page shows the detail of the calculations performed to estimate the general election result for 2017 had the new boundaries for Bromley and Chislehurst been in force at that time. The basic idea of the calculation is to look at the district council wards which make up the new seat, and estimate how they voted in 2017. This estimate is based on the recent local election results in those wards, with adjustments made to allow for different turnouts and different voting patterns for local and general elections.

In many cases, wards are divided between several old constituencies or between new constituencies. So the same ward may be shown more than once, but the electorate shown will reflect that ward sub-section.

Election results from a recent local election are given. This is usually the local election closest to 2015 from the period 2012-2015. For multiple-member wards, the votes shown are the sum of the votes cast for all candidates of each party. A negative number indicates candidate(s) elected unopposed.

Calculation method

There are two problems with using the raw (actual) local election results to imply general election results ward-by-ward. Firstly, turnout can be different between local and general elections, which means that the total number of local votes cast does not equal the number of votes cast in each old seat at the general election. To correct for this, we adjust the local votes to match the general election turnout. Each ward's result is scaled, whilst keeping constant the percentage support for each party, so that the total turnout adds up to the old seat's general election turnout. All wards in the old seat are assumed to have the same percentage turnout.

Secondly, even though we now have the correct number of votes in the old seat, the party totals will not match the general election result. Some parties do better at the local election than the general election, and others do vice versa. We call these strong and weak parties respectively. The next step is to transfer votes from strong parties to weak parties. This is done by taking votes proportionally away from strong parties and putting them in a transfer pool. For instance if a party got 12,500 local votes, but only 10,000 general election votes, it will lose 20% over all the wards. These votes in each ward will be put in the ward's transfer pool and allocated to the weak parties. Weak parties will get votes transferred to them in proportion to the votes they need over the old seat. For instance, if two weak parties need 4,000 and 1,000 votes respectively, the first party will get 80% of each ward's transfer pool and the second party will get 20% of each ward's transfer pool. The transfer of votes between parties is done using the Electoral Calculus Transition Model.

More details of the calculation formulas are available.

Bromley and Chislehurst: Map

List of old seats forming part of Bromley and Chislehurst

The old seat(s) needed are:

• Beckenham
• Bromley and Chislehurst

Old seat: Beckenham

The table shows the transfer-adjusted results for each ward in the old seat of Beckenham, as well as showing which new seat each ward is in.

The full details of these calculations are given on the Beckenham seat details page. Click on "Show workings".

Old seat: Bromley and Chislehurst

The table shows the transfer-adjusted results for each ward in the old seat of Bromley and Chislehurst, as well as showing which new seat each ward is in.

The full details of these calculations are given on the Bromley and Chislehurst seat details page. Click on "Show workings".

New seat: Bromley and Chislehurst

And these are the implied results for 2017 for the new seat of Bromley and Chislehurst. Please note that the wards used are those of 2015 in order to be consistent with those used by the Boundary Commissions.

© 2024 Electoral Calculus Ltd
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