After the first leaders debate there was a single poll showing Ed Miliband with a better approval rating that David Cameron. It produced a typical example of rubbish media handling of polls – everyone got all excited about one unusual poll and talked about it on news bulletins and so on, giving it far more prominence than the far greater number of polls showing the opposite. Nevertheless, it flagged up a genuine increase in Ed Miliband’s ratings.

I talk about leader ratings, but the questions different companies ask actually vary greatly. Opinium, for example, ask if people approve or disapprove of what each leader is doing, YouGov ask if they are doing a well or badly, ICM if they are doing a good or bad job. To give an obvious example of how these could produce different figures, UKIP have quadrupled their support since the election, so objectively it’s quite hard to argue that Nigel Farage hasn’t done well as UKIP leader…but someone who supported EU membership and freedom of movement probably probably wouldn’t approve of his leadership.

The graph below shows the net ratings for David Cameron and Ed Miliband from the four pollsters who ask leader ratings at least monthly (ComRes, ICM and Ashcroft all ask their versions of the question too, but not as frequently).

You can see there is quite a lot of variation between pollsters, but the trends are clear. Ed Miliband’s ratings have improved over the course of the campaign, though on most pollsters’ measures he remains significantly behind David Cameron. The main exception is Survation’s rating – I suspect this is because of the time frame of their question (Survation ask people to think specifically about the last month, other companies just ask in general – my guess is that the difference is people thinking Ed Miliband has done well in the campaign). Cameron’s ratings too have improved from three of the four pollsters, but not to the extent of Miliband’s.

What’s the impact of this? Theoretically I suppose it makes the potential for voters to be deterred from voting Labour by their lack of confidence in Ed Miliband that bit smaller. Whether that’s any real difference or not is a different matter – on one hand, while it is one of the Conservative party’s hopes that Miliband’s poor ratings will drive people towards voting Tory at the last minute, that’s very different to it actually happening. This could be the case of a window closing upon something that didn’t see to be happening anyway. The alternative point of view is that no one realistically expects some vast last swing producing a Tory landside – we are talking about grinding out a few percentage points at the margins. Hence Miliband’s ratings overall don’t necessarily make much difference – much of the increase is amongst Labour’s own voters anyway – it’s how he is seen amongst those small groups who are undecided whether or not to vote for Labour, and those who undecided whether or not to vote Tory to stop Miliband.

As we come to the final weeks of the election campaign, that’s a key to understanding a lot of polling. Most voters have already made their minds up (and even many of those who say they haven’t, are probably less likely to switch than they think they are). As postal votes have started to go out, an increasing number of people will have actually voted anyway. Most things that happen over the next two and a half weeks will have no impact on public opinion anyway – for those that do, it’s not national opinion that will make a difference, it’ll be the impact on that dwindling group of people who may yet be persuaded.