David Cowling looks at the track record of polls in previous elections. He explores why their results matter and if there were warning signs that indicated they were going wrong.

Last year's general election should have been an easy result to predict. There was a constant stream of opinion polls, many more than in previous campaigns. But they turned out to be highly misleading, suggesting a hung parliament. The actual result was a huge shock to the polling industry. So went wrong with the polls, and why? And how easy will it be to put it right?

In the first part of a series examining the role of opinion polling in British politics, David Cowling looks at the track record of polls in previous elections. He explores why their results matter and whether there were warning signs that should have indicated they were going wrong before last year's general election.

Producer: Martin Rosenbaum.