MRP and the Remain United Preamble

Multi-level regression and post-stratification (MRP) is a statistical technique used to project national polling data onto electoral districts, such as constituencies or regions. This is important for understanding elections where the geography of the nationwide vote is crucial, such as states in the US (for their electoral college), or constituencies in the House of Commons.

Remain United commissioned an MRP projection, using internet panel polling data from ComRes, for European Parliament voting intention. ComRes interviewed 4,060 GB adults between 1st and 7th May 2019.

The statistical outputs were then used to suggest tactical voting. This article considers some issues with the supporting document.

Response Rates and Non-Response Bias

The document starts by recounting that the last two general elections and the EU referendum “were not predicted well by most pollsters”.

Industry response rates are now very low, with most people refusing to take part. Those who do participate are unusual, and the sample becomes unrepresentative, which creates inaccuracy.

Non-response bias is an issue in survey research where the respondents are substantially different from those who were selected but failed to respond.

It is then asserted:

They might have worked well in the past when samples were only a little skewed, but they groan under the weight of modern…