Election Polls, Online

The fact that fewer people own a landline could affect phone polling in future but current surveys do speak to some participants on their mobiles. Photograph: Dave Thompson/PA

There is another clear divide in this election, not only the one between Labour and the Conservatives. There is also a stark difference between telephone and internet polling.

Related: Conservatives take six-point lead in Guardian/ICM poll

A series of internet polls last week had Labour’s lead ranging from three to six points. But a ComRes phone poll released a few hours later had the Tories ahead, albeit by one point.

Some of these differences can be explained by the standard margin of error of +/- three points, due to random sampling error, which all polls have to deal with.

However, looking at the trends within internet and phone polls reveals important differences.

The current 10-day average among telephone polls has the Tories on 35.5%, Labour on 33.5%, the Lib Dems on 8.5%, Ukip at 11% and the Greens on 6%.

The average of online polls has the Conservatives (32%) trailing Labour (34%). The Lib Dems, on 8%, are on more or less the same level of support with both sets of pollsters. Ukip are on 15.5% among online firms, and the Greens on 4.5%.

In both averages, figures are adjusted to take into account the frequency of polling and each pollster’s past performance.

The difference in the numbers for Nigel Farage’s and Natalie Bennett’s parties between phone and web are particularly significant. That’s because any drop in Ukip support is more likely to benefit David Cameron’s electoral chances, while the Greens’ share of the vote influences Labour’s chances in tens of constituencies.

This means that in polls that show lower Ukip support and higher figures for the Greens, the Tories are more likely to benefit over Labour. And where the opposite (higher Ukip, lower Greens) is the case, Labour is more likely to be the beneficiary.

So, which method is better?

The problem is that we won’t know for sure until 8 May.

In 2010 phone polls were in general closer to the final outcome – but this was primarily driven by internet surveys overestimating support for the Lib Dems.

Most internet pollsters were broadly correct when it came to figures for the two largest parties. YouGov’s final figures in 2010 had the Tories on 35% and Labour on 28%, one point off their final results (36% and 29% respectively).