Why 4 is a magic number

March 26, 2010

David Cameron has waved goodbye to his chances of becoming Prime Minister, a poll in the Daily Mirror claimed earlier this week.

The poll, carried out by Ipsos Mori, puts the Tories on 35% with Labour on 30% and Lib Dems on 21%. Because of the peculiarities of the British electoral system, this result would give Gordon Brown about 290 MPs, making Labour the largest single party but with no overall majority.

All well and good. But there is one fact missing: the margin of error. The full report from the Ipsos Mori website makes no secret of the fact that the margin of error is 4% (although this figure is nowhere to be found in the Mirror). Since the putative difference between the parties is only 5%, that’s a pretty significant margin. As Ipsos Mori say about the margin of error: “This is especially important to keep in mind when calculating party lead figures”. Seems the Mirror forgot to read this bit.

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