Research
How the forecast works, in full
The 10 papers behind our Spain forecast (20,489 words): how far the polls have missed, how we correct them, the seat model and the tests.
These papers set out every part of our method: what we measure, how we correct the polls, how votes become seats, how uncertain the result is, and what we have tested and rejected. Figures marked Live are computed from the database each time the page is built, so they always match the forecast. En español
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10 papers · about 20k words · Our method, in short · How the elections work · Data sources
Spain
- Forecasting the Congreso from imperfect polls: an overview The Spain section forecasts the next general election with the rule used for Great Britain: take half the average past miss off today's polls, but only where that would have improved past forecasts made without knowing the result. Spanish final polls fail that test and are taken as published. The seats are decided by D'Hondt in 52 provinces, most of them small, and our calculation reproduces all 350 seats of 2023. This paper sets out the evidence, the seat model, the sources of uncertainty and what the forecast does not claim, and maps the other Spain papers.
- Measuring Spanish polling error, 2004–2023 We measure the error of the final polls at the eight Spanish general elections from 2004 to 2023, taken under the five-day ban on publishing polls: which polls count, how their sources are graded, the error party by party and on the gap between the two largest parties, each pollster's record and the long view back to 1977. The left of the PSOE is measured as one lineage. The two largest misses, in 2004 and 2016, were shared by every pollster; no party has been missed the same way at every election.
- Reading Spanish polls: the correction test and the CIS's raw figures The core methods paper for Spain. Taking off half the average past miss, with each election predicted from the others, was further from the result at most of the eight general elections since 2004, so Spanish polls are taken as published. The CIS's direct vote, rescaled to those naming a party, was further out than its own estimate at five of seven elections, so we use the estimates. The paper also summarises the seat calculation, which reproduces the 2023 Congreso exactly.
- From votes to seats: D'Hondt in 52 provinces We implement the LOREG's seat rules: 350 seats, one each for Ceuta and Melilla, the rest apportioned to provinces with a minimum of two, and D'Hondt within each province among lists with 3% of the valid vote. Given the official votes, the calculation reproduces all 350 seats of 2023, and every election since 2004. D'Hondt in small provinces gives the largest parties a bonus and leaves middling national parties short. National polls reach the provinces by proportional swing from 2023, with Sumar's 2023 vote split between Sumar and Podemos; this projection has not been backtested.
- Simulating Spanish polling error: where the chances come from The seat ranges, each party's chance of winning a seat and the chance of each combination reaching 176 come from 2,000 simulated elections. Each party's national share is moved by a random error the size of its typical final-poll miss, each party's share in each province by a further local factor, and D'Hondt is run in every province. We explain the spreads used, including for Sumar, Podemos and parties with no record, and what independent draws leave out.
- From today to polling day in Spain We test the British pull-back towards the previous result on Spanish polls at six distances up to a year before the general elections of 2011 to 2023. It did not beat the polls as they stood at any distance, because change between elections has been large and has lasted. The polling-day forecast therefore keeps today's shares and widens the ranges by a drift fitted to how far polls have missed at each distance.
- If we're wrong: replaying past Spanish polling misses on today's polls Ten readings of today's Spanish polls, side by side: the polls as they stand, our projection (the same, since correcting failed its test) and each general election's final-poll miss since 2004 replayed on today's average, all through the same provincial seat engine. The 2004 replay moves the Congreso most; D'Hondt makes the split left's seats jump. We also explain the count of provinces at risk.
- Rating Spanish pollsters: accuracy, the lead and openness Each Spanish pollster is rated out of 100 on the British scale: accuracy at past final polls, the error on the gap between the two leading parties, and how much it publishes. Records are pulled towards a poor Spanish final poll. The CIS, a public body, is rated like everyone else; its direct vote was further from the result than its own estimate at five of seven elections, which is why we do not start from raw figures.
- The Spanish data: results, maps, statistics, deputies and polls Where every number on the Spanish pages comes from: the Junta Electoral Central's results in the BOE, the IGN's boundaries, the INE's statistics, the CIS's studies, pollsters' and newspapers' releases, and Wikipedia's tables where nothing else could be read. How each was changed and checked, what is left out and why, the daily poll check, the licences, and the known gaps, including the Congreso photographs we do not use.
- What we don't know yet: limitations of the Spanish forecast Every known weakness of the Spanish forecast in one place: eight elections with two exceptional misses, the last seat in small provinces, independent errors in the simulation, untested proportional swing with no regional layer, the split left, new and local parties, the election date, second-hand polls and the gap between seats and investiture. For each, how much it matters and what evidence would change our mind.
