Basics and Caveats

Some of the work I do here on Ballot Box Scotland relates to what are known as “projections”. Projection involves taking a national poll of voting intention and seeking to translate it to a sub-national level. For obvious reasons, people are usually keen to have an indication of what a given poll suggests for their area and, in the case of parliamentary polling, how many seats each party would win.

The most common form of projection is called “Uniform National Swing” (UNS), and it works on the basis that if a party shows a 1% increase in a poll, that will be a 1% increase everywhere in the country. Combined with the results of the last election, UNS can then be used to project the winning party in each constituency.

Of course the reality is that isn’t how elections actually work. Party strength varies across the country such that that 1% increase might be 2% in an area they are typically strong in, but just 0.1% in a weaker area.

Further, a party that is completely static versus their last election result overall can experience substantial swings in individual areas. The Lib Dems evidenced this between 2011 and 2016 by winning basically the same vote shares overall, but winning 2 constituencies and losing 25 deposits in 2011, then winning 4 constituencies but losing 48 deposits in 2016.

Some forms of projection try and account for such known distributions of support. Others don’t go for simple declarations of which party will win a given seat, and instead operate on the basis of a % probability of each party winning it.

Even with those more nuanced forms of projection, local issues, personalities, and campaigns have a substantial impact, as do changing demographics. So it simply isn’t possible to take a national poll and use it to predict with perfect accuracy the result in each constituency. Projections are useful indications and a lot of fun, but not reliably more than that.

This should be quite obvious, and the Ballot Box Scotland ethos is to try and improve understanding of elections whilst also assuming people know enough to start with that they don’t need to be beaten over the head with caveats and health warnings every time I post something. Good plain fun is also part of the ethos.

Not doing something lots of people enjoy and find informative because some people might occasionally misinterpret or misunderstand it isn’t in keeping with that ethos. And that’s where this page comes in – a quick and easy explanation of some of the issues with projection, just in case people do need it.

Scottish Parliament

Holyrood projections are one of my bread and butter bits of content, and amongst the most popular. With at least five parties in play for seats in each poll and substantial regional variance in support, UNS is definitely not going to be much use. Instead, I’ve got a home-brew form of projection I’ve dubbed “Regionally Moderated Swing.”

This still uses UNS as a starting point, but combines it with a calculation that accounts for regional distribution of support. This calculation assumes that if, say, 10% of a party’s votes came from one region last time, then they’ll be similarly distributed this time. UNS and the results of this calculation are averaged out, so that the Lib Dems don’t just eat the Highlands, then there’s a little bit of tweaking to prevent any region ending up with parties totalling 112% of the vote. A roughly similar calculation happens at constituency level too.

Again, this is absolutely not a perfect approach. A party doing very well in a region last time might suggest they’ve hit saturation point there and they won’t grow much further even with a big positive national swing, or that even with a small negative national swing they’ve got a lot further to fall there than in regions they might already be rock bottom. Still, by assuming the SNP to be undergoing a support shift from the North East to the Central Belt, the Conservatives to be strong in rural areas vs Labour in urban, Greens in the two big cities, and Lib Dems in the Highlands, it’s hopefully better than UNS.

To further emphasise the fuzzy nature of projections, for the Constituencies I also provide a map showing the runner-up in each, and one showing what band the winning margin falls into. Effectively, you can assume that a constituency in the 20%+ projected margin band is highly likely to go the way the main map indicates, whereas one in the 0-5% band could still very easily actually be won by the projected runner-up.

UK Parliament

As I explain in detail in this post, I don’t do projections for Westminster. The short version is as follows;

  • Westminster is widely covered by others as it is
  • Pure First Past the Post is difficult to reliably project
  • First Past the Post projections dramatise small shifts in vote share yet are intensely boring due to non-impact of new and/or smaller parties
  • I don’t have an in-house calculator for Westminster, and don’t want to develop one for the reasons above


Independence projections are now only done for annual reviews, though they were previously a normal part of polling coverage. These are simple UNS, as described above, and are even more “just for fun” than other projections. In reality, even with an identical national result to 2014, the distribution of votes between areas would have some difference.

In particular, following the Brexit Referendum, there has been some degree of alignment between Yes/Remain and No/Leave. Areas with higher than average 2016 Remain votes are therefore likely to be more supportive of Independence than in 2014 even on an equivalent national vote share. On the other hand, areas with higher than average 2016 Leave votes are likely to be more supportive of the Union than in 2014 in those circumstances.