Studying specific seats
Turning to seat projections, on a uniform national swing, the May2015 polling averages would give the Lib Dems 23 MPs. But with such a substantial swing against them, the uniformity of swings around Great Britain bears scrutiny. As May2015 detailed last week, the Lib Dem-to-Labour swing is far greater than the Lib Dem-to-Conservative swing.
By examining the constituency voting question in the most recent Lib Dem battleground Ashcroft poll, we can see that the former swing is some 1.5 to 2 per cent less than in the Ashcroft National Polls and the latter 2.5 to 3 per cent more than nationally. I’ve excluded Eastleigh from these calculations, due to the risk that the 2013 by-election (and disproportionate campaigning) in the Hampshire constituency has made it unrepresentative of swings in other Con-Lib contests.
But it’s not just the party in second place that incumbents should fear. In Scotland the national SNP surge is so great that if repeated uniformly across Scotland, it would unseat an additional 2 Lib Dem MPs in seats where the SNP wasn’t even runner-up in 2010, and come close to unseating a third.
It would also change the winner of several seats that I already have the Lib Dems losing. Given the size of the moves, uniformity is even less likely across Scotland than south of the border, which puts all the more meaning on Lord Ashcroft’s inaugural Scottish constituency polls, due next month.
In terms of Lib Dem seats where nationalist parties actually came second in 2010, Gordon looks very likely to fall to the SNP. In Wales, Ceredigion will likely be more of a battle, but none of the (admittedly limited) Welsh polling thus far suggests sufficient gains by Plaid Cymru to win it, so I’ve marked it as a Lib Dem hold.
The May2015 averages show national swings against the Lib Dems of 9.5 per cent to Labour and 5.5 per cent to the Conservatives. Applying the differentials above (and dividing them by 2), these become 12.2 per cent and 3.9 per cent respectively. Applying these differential swings (and the SNP adjustment) across the battlegrounds saves the Lib Dems 5 seats in contests with the Conservatives but costs them 2 in defences from Labour and 2 to the SNP that they would otherwise have held, resulting in a net saving of one seat.
Why use Lord Ashcroft’s constituency polling in this way instead of just taking the results at face value, given that individual seats often behave differently to the national or even battleground picture? First of all, there are very few pollsters (in most cases, just one) polling each constituency, so the impact of those pollsters’ house effects is far greater than in national polls.
As mentioned above, Ashcroft polling tends to show lower-than-average support levels for Labour and the Conservatives and relatively high figures for the Lib Dems. This is why the 3.9 per cent swing to the Tories that I’m working with is larger than the 2 per cent taken directly from the poll. Comparing constituency polls with national polls by the same pollster “nets out” these effects to a large extent, save for any “in-house” house effect between national and constituency polling.
Secondly, constituency polling tends to be published with a lag, as the various constituencies aren’t all polled concurrently. Things have changed in national polling since the fieldwork took place in August and September, with Labour dropping several points. This is enough to offset the Lib-Lab Ashcroft house effect, so in Lib-Lab seats my 12.2 per cent swing is basically the same as in the seat polls.
Thirdly, sample size is an issue. The Ashcroft battleground polls collectively have a massive sample (22,000 in the latest), but in individual constituencies it is only 1,000, as in national polls.
The margin of error for the spread between two parties on around 32 per cent of the vote is about ±1 point for the full sample or ±2 points for the 5 LIB-LAB contests, but ±5 points for each individual seat’s thousand-voter sample.
Of the 22 seats polled, 12 had majorities smaller than that, a further 7 had statistically significant majorities but the aggregate swing correctly predicted the winner, while the 3 “surprises” split 2-1 between holds and losses respectively.
So while constituency polls provide us with extremely useful data, they are still opinion polls and there are statistical benefits to aggregating them, particularly where we are looking at overall seat numbers rather than specific constituencies. And as Lord Ashcroft himself says repeatedly, polls are snapshots, not predictions.