2010 Election polls

David Cameron, the prime minister, and Labour party leader Ed Miliband Photograph: PA/PA

On Saturday evening, the Labour flank of the Twittersphere erupted in jubilation as a YouGov poll placed Ed Miliband’s party four points ahead of the Conservatives.

Latest YouGov poll (27 Mar): LAB - 36% (+2) CON - 32% (-4) UKIP - 13% (-) LDEM - 8% (+1) GRN - 6% (+1)

Latest ComRes poll (28 - 29 Mar): CON - 36% (+1) LAB - 32% (-3) UKIP - 12% (+2) LDEM - 9% (+1) GRN - 5% (-2)

Labour’s joyous bubble burst, the Tories exulted.

Over the next five weeks this pattern will be repeated many more times as the number of polls published increase in the runup to 7 May.

This raises two questions.

How can two polls be so different?

Polls carry a margin of error and levels of confidence. For example, a poll of 1, 000 people has a margin of error of about plus or minus three points and a confidence interval of 95%. In theory, this means 95 times out of 100 the figure in one poll will be within three percentage points of what it would be if you surveyed the entire population.

This implies that any change within the margin of error isn’t significant, and that there is always the possibility of a random error. There is always an element of uncertainty.

One way to alleviate this is to look at the trend across polls, instead of the one set of figures that we like best.

It may well be that either the YouGov or the ComRes poll are reflecting the emergence of a new trend, but based on current evidence it’s more likely that the Conservatives and Labour remain virtually tied.

Can we trust polling?

The short answer is yes.

Within the context of uncertainty, polls remain the most reliable method of assessing public opinion. For example, when comparing the 2010 polls with the final election result, the polling was quite accurate:

The challenge is how we read and understand polling – and how we deal with and explain uncertainty.

When there are 10 polls, and nine tell the same story but one looks very different, however tempting it is to focus on the one, the other nine are probably showing the more accurate picture – probably, not definitely.

What are the polls saying today?

Both main parties remain well short of an outright majority.

One number to keep an eye on over the next five weeks is the combined Labour-SNP share of seats. It’s currently projected at 322 seats – anything above this, and Cameron wouldn’t have the numbers to form a government if the MPs of both parties were to vote him down.