A recent Opinium poll published a tie in vote intentions for the main parties. The earlier YouGov poll estimated a seven-point Conservative lead over Labour. Both internet panel polls ran on similar dates: 24–25th August for YouGov, and 26–28th August for Opinium.
This article looks at estimating systematic differences between polling companies.
On the house
Researchers in political science may refer to polling companies as ‘houses’.
The choices that market research companies can affect their published vote intentions. A house effect is the systematic difference arising from those choices in methods.
When compared to actual election results, we find the ‘house bias’. **This is not about political bias or partisanship. **Unlike other countries, polling companies in Britain do not align to political parties. There are no Conservative companies and Labour companies.
Major market research companies sign up to the Market Research Society’s code of conduct. There are the British Polling Council transparency rules.
There are choices researchers take, which can affect headline estimates:
- Target population: Some companies survey the whole United Kingdom. Others may survey adults in Great Britain.
- Survey mode: Social desirability can influence responses to surveys with interviewers. Internet panels can vary in quality.
- Question wording and order: Researchers write their respective question in different ways. The 2015 polling inquiry concluded question wording and order did not add to the miss.
- Response options: Companies choose which parties to offer as answers to their question. There are often differences in prompts for new parties.
- **Weighting: **The weights affect how the final sample looks like the population. Using recalled past votes to weigh samples can lead to issues with false recall. Different companies can use different weighting targets.
- Turnout modelling: There are many different ways to move from a sample of adults to voters.
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