Two new polls out today, both good for Labour. Populus this morning had toplines of CON 31%, LAB 38%, LDEM 9%, UKIP 14% (tabs here). Lord Ashcroft’s weekly poll has topline figures of CON 27%, LAB 34%, LDEM 11%, UKIP 15% (tabs here).

The Ashcroft poll comes after a poll last week that showed the Conservatives 2 points ahead, and has naturally provoked some comment about volatility. In one sense it’s fair comment – Ashcroft polling has been volatile. In another sense it’s not – Ashcroft’s polling hasn’t necessarily been any more volatile than you should expect, it’s just that we sometimes have slightly unrealistic expectations of how accurate a poll of 1000 people should be!

The standard margin of error on a poll of 1000 people is plus or minus 3 points. However, voting intention figures aren’t based on the whole sample, only on those who give a voting intention – in a phone sample of 1000 that’s typically 500 or so people, giving a margin of error of plus or minus 4 points. I should add that the margin of error is based upon what the margin would be in a pure random sample. This is very much a polite fiction – no voting intention polls are actual pure random samples. Many are from internet panels, even quasi-random phone polls aren’t actually random because of low response rates. Weighting effects would also change the actual margin of error.

Looking at Ashcroft’s nine regular polls to date, the average level of Labour support has been 33%, and all nine polls have been within 2 points of this. The average Lib Dem support has been 8.5%, and all nine polls have been within 2.5% of this. What’s made them look erratic is the level of Tory support, which has averaged 29%, but has varied between 25% and 34% – two of Ashcroft’s Tory scores have differed from the average by 4 points, one by 5 points. This assumes that there hasn’t been any genuine movement in Tory support, when it’s possible there has. Ashcroft’s highest Tory score came in his first poll in mid-May, at a time when ICM also showed a Tory lead and YouGov a neck-and-neck. Ashcroft’s lowest Tory score came just after the European results when UKIP had a post-European election boost.

Bottom line is that while Ashcroft’s polls look erratic, they probably aren’t much more erratic than we should expect from topline figures based on 500 people. There isn’t anything strange about their methodology, nothing odd going on, it’s just the normal limits of how precise polling with a given sample size can be. And it’s a useful reminder of why we shouldn’t read too much into individual polls, and it’s the underlying trend and average that count.

This morning’s YouGov poll for the Sun had topline figures of CON 35%, LAB 36%, LDEM 8%, UKIP 12%. Tabs are here. The twice-weekly Populus poll also had a one point lead today, CON 34%, LAB 35%, LDEM 9%, UKIP 14%. Tabs are here.

Polls do seem to be showing some narrower Labour leads this week, though really too early to tell if it’s anything meaningful. YouGov definitely seem to be picking up less UKIP support post-Juncker (last week they had UKIP in the range 13%-15%, this week 11%-12%), but the same trend isn’t obvious in Populus’s polls this week.

When you sit down to do a marginals polls one consideration is where you draw the line: what is a marginal? The thing you want to avoid is under or overshooting the real battleground – the risk is that you poll lots of seats that need a swing of up to 5% and find a swing of 10%, enough to win lots of seats you didn’t bother polling.

That’s the sort of thing that’s happened in the third of Lord Ashcroft’s three sets of marginal polls – full details here. He polled the four most marginal LD -v- Lab seats, Norwich South, Bradford East, Brent Central and Manchester Withington. These need a swing of up to 2.1% to go from LD to Lab, which unsurprisingly Labour get easily. The average LD => Lab swing in these seats was 15%, confirming that the Lib Dems are doing much worse where they are up against Labour and easily enough to unseat almost all Lib Dem MPs with Labour in second place. In practice of course we can’t actually be that confident that voters in a tight LD-Lab marginal will behave the same way as in a seat where the Lib Dems have a 20% majority, so it’s a bit of a shame Ashcroft didn’t include some more challenging LD-Lab fights like Cambridge, Hornsey & Wood Green or Bermondsey.

An interesting thing to note is that the Greens are doing notably well in a couple of these seats. In Norwich South they are in second place on 20%, but that was one of their target seats anyway, they are also doing well in Manchester Withington, up 8 points on 10%.

While it’s hardly a LD-Lab battle, Lord Ashcroft also polled Brighton Pavilion, Caroline Lucas’s seat. Voting intentions there with changes from the general election are CON 18%(-6), LAB 33%(+4), LDEM 5%(-9), GREEN 32%(+1), suggesting an extremely tight race between Labour and the Green party.