New Seat Details - Rhondda and Llantrisant

This page shows the detail of the calculations performed to estimate the general election result for 2017 had the new boundaries for Rhondda and Llantrisant 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.

Party	Implied 2017 Votes	Implied 2017 Share
LAB	31,928	60.9%
Plaid	9,615	18.3%
CON	8,455	16.1%
UKIP	1,493	2.8%
LIB	922	1.8%
LAB Majority	22,313	42.6%

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.

Rhondda and Llantrisant: Map

List of old seats forming part of Rhondda and Llantrisant

Old seat: Ogmore

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

Old seat: Ogmore				Transfer-adjusted Results
District	Ward	Electorate 2017	New Seat	CON Votes	LAB Votes	LIB Votes	UKIP Votes	Green Votes	Plaid Votes	MIN Votes	OTH Votes	Total Votes
Bridgend	Aberkenfig	1,692	Ogmore and Aberavon	289	494	17	38	0	172	0	0	1,010
Bridgend	Bettws (Ogmore)	1,536	Ogmore and Aberavon	215	815	8	33	0	38	0	0	1,109
Bridgend	Blackmill	1,870	Ogmore and Aberavon	279	857	12	36	0	53	0	0	1,237
Bridgend	Blaengarw	1,260	Ogmore and Aberavon	187	619	17	29	0	40	0	0	892
Bridgend	Bryncethin	995	Ogmore and Aberavon	162	413	26	20	0	44	0	0	665
Bridgend	Bryncoch	1,652	Ogmore and Aberavon	264	738	34	34	0	49	0	0	1,119
Bridgend	Caerau	4,593	Ogmore and Aberavon	726	2,492	29	121	0	117	0	0	3,485
Bridgend	Cefn Cribwr	1,088	Bridgend and Vale of Glamorgan West	186	483	5	23	0	42	0	0	739
Bridgend	Felindre	2,046	Ogmore and Aberavon	402	822	51	43	0	63	0	0	1,381
Bridgend	Hendre	2,985	Ogmore and Aberavon	631	1,243	21	64	0	98	0	0	2,057
Bridgend	Llangeinor	846	Ogmore and Aberavon	151	418	4	19	0	28	0	0	620
Bridgend	Llangynwyd	2,330	Ogmore and Aberavon	445	788	16	54	0	214	0	0	1,517
Bridgend	Maesteg East	3,536	Ogmore and Aberavon	564	1,506	18	84	0	206	0	0	2,378
Bridgend	Maesteg West	4,185	Ogmore and Aberavon	684	1,700	90	100	0	316	0	0	2,890
Bridgend	Nant-y-moel	1,657	Ogmore and Aberavon	297	756	19	42	0	53	0	0	1,167
Bridgend	Ogmore Vale	2,193	Ogmore and Aberavon	394	982	36	54	0	88	0	0	1,554
Bridgend	Penprysg	2,337	Ogmore and Aberavon	443	949	24	49	0	100	0	0	1,565
Bridgend	Pontycymmer	1,648	Ogmore and Aberavon	256	876	19	42	0	52	0	0	1,245
Bridgend	Sarn	1,748	Ogmore and Aberavon	270	840	16	38	0	54	0	0	1,218
Bridgend	Ynysawdre	2,555	Ogmore and Aberavon	460	935	21	57	0	209	0	0	1,682
Rhondda Cynon Taf	Brynna	3,264	Ogmore and Aberavon	522	1,075	18	61	0	228	0	0	1,904
Rhondda Cynon Taf	Gilfach Goch	2,411	Ogmore and Aberavon	366	1,196	48	52	0	62	0	0	1,724
Rhondda Cynon Taf	Llanharan	2,610	Ogmore and Aberavon	498	966	15	55	0	194	0	0	1,728
Rhondda Cynon Taf	Llanharry	2,940	Rhondda and Llantrisant	491	1,070	25	58	0	203	0	0	1,847
Rhondda Cynon Taf	Pont-y-clun	707	Rhondda and Llantrisant	168	190	6	29	0	71	0	0	464
Rhondda Cynon Taf	Talbot Green	12	Rhondda and Llantrisant	2	4	0	0	0	1	0	0	7
	Total	54,696		9,352	23,227	595	1,235	0	2,795	0	0	37,204

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

Old seat: Pontypridd

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

Old seat: Pontypridd				Transfer-adjusted Results
District	Ward	Electorate 2017	New Seat	CON Votes	LAB Votes	LIB Votes	UKIP Votes	Green Votes	Plaid Votes	MIN Votes	OTH Votes	Total Votes
Rhondda Cynon Taf	Beddau	3,090	Rhondda and Llantrisant	480	1,282	65	48	0	192	0	0	2,067
Rhondda Cynon Taf	Church Village	3,469	Cynon Valley and Pontypridd	710	1,384	87	55	0	262	0	0	2,498
Rhondda Cynon Taf	Graig (Pontypridd)	1,455	Cynon Valley and Pontypridd	265	777	56	29	0	94	0	0	1,221
Rhondda Cynon Taf	Hawthorn	2,869	Cynon Valley and Pontypridd	393	970	124	46	0	245	0	0	1,778
Rhondda Cynon Taf	Llanharry	94	Rhondda and Llantrisant	15	37	2	1	0	7	0	0	62
Rhondda Cynon Taf	Llantrisant Town	3,590	Rhondda and Llantrisant	665	1,366	71	56	0	257	0	0	2,415
Rhondda Cynon Taf	Llantwit Fardre	4,593	Cynon Valley and Pontypridd	1,345	1,216	86	70	0	314	0	0	3,031
Rhondda Cynon Taf	Pont-y-clun	5,888	Rhondda and Llantrisant	1,157	1,435	113	186	0	598	0	0	3,489
Rhondda Cynon Taf	Pontypridd Town	2,141	Cynon Valley and Pontypridd	403	698	124	37	0	142	0	0	1,404
Rhondda Cynon Taf	Rhondda	3,364	Cynon Valley and Pontypridd	554	1,425	95	60	0	179	0	0	2,313
Rhondda Cynon Taf	Rhydfelen Central / Ilan	2,924	Cynon Valley and Pontypridd	385	1,533	103	46	0	153	0	0	2,220
Rhondda Cynon Taf	Taffs Well	2,758	Cardiff West	454	1,004	67	46	0	266	0	0	1,837
Rhondda Cynon Taf	Talbot Green	1,936	Rhondda and Llantrisant	359	754	44	34	0	108	0	0	1,299
Rhondda Cynon Taf	Ton-Teg	3,170	Cynon Valley and Pontypridd	623	1,020	165	50	0	132	0	0	1,990
Rhondda Cynon Taf	Tonyrefail East	4,215	Rhondda and Llantrisant	725	1,681	168	80	0	284	0	0	2,938
Rhondda Cynon Taf	Tonyrefail West	4,620	Rhondda and Llantrisant	719	1,851	100	82	0	408	0	0	3,160
Rhondda Cynon Taf	Trallwng	2,770	Cynon Valley and Pontypridd	489	960	223	49	0	151	0	0	1,872
Rhondda Cynon Taf	Treforest	1,845	Cynon Valley and Pontypridd	574	1,547	219	57	0	171	0	0	2,568
Rhondda Cynon Taf	Tyn-y-nant	2,465	Rhondda and Llantrisant	340	1,163	51	38	0	137	0	0	1,729
	Total	57,256		10,655	22,103	1,963	1,070	0	4,100	0	0	39,891

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

Old seat: Rhondda

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

Old seat: Rhondda				Transfer-adjusted Results
District	Ward	Electorate 2017	New Seat	CON Votes	LAB Votes	LIB Votes	UKIP Votes	Green Votes	Plaid Votes	MIN Votes	OTH Votes	Total Votes
Rhondda Cynon Taf	Cwm Clydach	1,974	Rhondda and Llantrisant	124	906	9	35	0	248	0	0	1,322
Rhondda Cynon Taf	Cymmer	3,889	Rhondda and Llantrisant	241	1,815	16	68	0	465	0	0	2,605
Rhondda Cynon Taf	Ferndale	2,953	Rhondda and Llantrisant	222	1,328	23	59	0	345	0	0	1,977
Rhondda Cynon Taf	Llwyn-y-pia	1,594	Rhondda and Llantrisant	98	691	12	28	0	239	0	0	1,068
Rhondda Cynon Taf	Maerdy	2,234	Rhondda and Llantrisant	115	1,135	6	38	0	202	0	0	1,496
Rhondda Cynon Taf	Pen-y-graig	3,925	Rhondda and Llantrisant	271	1,671	18	66	0	604	0	0	2,630
Rhondda Cynon Taf	Pentre	3,690	Rhondda and Llantrisant	298	1,354	20	69	0	731	0	0	2,472
Rhondda Cynon Taf	Porth	4,209	Rhondda and Llantrisant	320	1,682	30	76	0	714	0	0	2,822
Rhondda Cynon Taf	Tonypandy	2,649	Rhondda and Llantrisant	192	1,139	17	47	0	381	0	0	1,776
Rhondda Cynon Taf	Trealaw	2,854	Rhondda and Llantrisant	195	1,342	15	50	0	310	0	0	1,912
Rhondda Cynon Taf	Treherbert	4,048	Rhondda and Llantrisant	238	1,585	13	77	0	799	0	0	2,712
Rhondda Cynon Taf	Treorchy	5,438	Rhondda and Llantrisant	435	2,039	20	102	0	1,047	0	0	3,643
Rhondda Cynon Taf	Tylorstown	3,213	Rhondda and Llantrisant	150	1,686	7	49	0	260	0	0	2,152
Rhondda Cynon Taf	Ynyshir	2,346	Rhondda and Llantrisant	157	996	16	43	0	360	0	0	1,572
Rhondda Cynon Taf	Ystrad	4,145	Rhondda and Llantrisant	278	1,726	55	74	0	644	0	0	2,777
	Total	49,161		3,334	21,095	277	881	0	7,349	0	0	32,936

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

New seat: Rhondda and Llantrisant

District	Ward	Electorate 2017	Old Seat	CON Votes	LAB Votes	LIB Votes	UKIP Votes	Green Votes	Plaid Votes	MIN Votes	OTH Votes	Total Votes
New seat: Rhondda and Llantrisant
Rhondda Cynon Taf	Beddau	3,140	Pontypridd	480	1,282	65	48	0	192	0	0	2,067
Rhondda Cynon Taf	Cwm Clydach	2,028	Rhondda	124	906	9	35	0	248	0	0	1,322
Rhondda Cynon Taf	Cymmer	3,996	Rhondda	241	1,815	16	68	0	465	0	0	2,605
Rhondda Cynon Taf	Ferndale	3,034	Rhondda	222	1,328	23	59	0	345	0	0	1,977
Rhondda Cynon Taf	Llanharry	2,813	Ogmore	491	1,070	25	58	0	203	0	0	1,847
Rhondda Cynon Taf	Llanharry	94	Pontypridd	15	37	2	1	0	7	0	0	62
Rhondda Cynon Taf	Llantrisant Town	3,665	Pontypridd	665	1,366	71	56	0	257	0	0	2,415
Rhondda Cynon Taf	Llwyn-y-pia	1,638	Rhondda	98	691	12	28	0	239	0	0	1,068
Rhondda Cynon Taf	Maerdy	2,295	Rhondda	115	1,135	6	38	0	202	0	0	1,496
Rhondda Cynon Taf	Pen-y-graig	4,033	Rhondda	271	1,671	18	66	0	604	0	0	2,630
Rhondda Cynon Taf	Pentre	3,791	Rhondda	298	1,354	20	69	0	731	0	0	2,472
Rhondda Cynon Taf	Pont-y-clun	5,297	Pontypridd	1,157	1,435	113	186	0	598	0	0	3,489
Rhondda Cynon Taf	Pont-y-clun	707	Ogmore	168	190	6	29	0	71	0	0	464
Rhondda Cynon Taf	Porth	4,325	Rhondda	320	1,682	30	76	0	714	0	0	2,822
Rhondda Cynon Taf	Talbot Green	1,972	Pontypridd	359	754	44	34	0	108	0	0	1,299
Rhondda Cynon Taf	Talbot Green	12	Ogmore	2	4	0	0	0	1	0	0	7
Rhondda Cynon Taf	Tonypandy	2,722	Rhondda	192	1,139	17	47	0	381	0	0	1,776
Rhondda Cynon Taf	Tonyrefail East	4,460	Pontypridd	725	1,681	168	80	0	284	0	0	2,938
Rhondda Cynon Taf	Tonyrefail West	4,800	Pontypridd	719	1,851	100	82	0	408	0	0	3,160
Rhondda Cynon Taf	Trealaw	2,933	Rhondda	195	1,342	15	50	0	310	0	0	1,912
Rhondda Cynon Taf	Treherbert	4,159	Rhondda	238	1,585	13	77	0	799	0	0	2,712
Rhondda Cynon Taf	Treorchy	5,588	Rhondda	435	2,039	20	102	0	1,047	0	0	3,643
Rhondda Cynon Taf	Tylorstown	3,301	Rhondda	150	1,686	7	49	0	260	0	0	2,152
Rhondda Cynon Taf	Tyn-y-nant	2,626	Pontypridd	340	1,163	51	38	0	137	0	0	1,729
Rhondda Cynon Taf	Ynyshir	2,411	Rhondda	157	996	16	43	0	360	0	0	1,572
Rhondda Cynon Taf	Ystrad	4,259	Rhondda	278	1,726	55	74	0	644	0	0	2,777
	Total	80,099		8,455	31,928	922	1,493	0	9,615	0	0	52,413

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

Implied MP at 2017:	Chris Bryant (LAB)
County/Area:	Mid Glamorgan (Wales)
Electorate:	80,099
Turnout:	65.4%

Rhondda and Llantrisant: New Boundaries 2018 Calculation

Rhondda and Llantrisant: Overview

Introduction