Reader Steve M. noticed an oversight in the Guardian in the following bar chart (link):

Guardian_Brexitpoll_1

The reporter was discussing an important story that speaks to the need for careful polling design. He was comparing two polls, one by Ipsos Mori, and one by YouGov, that estimates the vote support for each party in the future U.K. general election. The bottom line is that the YouGov poll predicts about double the support for the Brexit Party than the Ipsos-Mori poll.

The stacked bar chart should only be used for data that can be added up. Here, we should be comparing the numbers side by side:

Redo_junkcharts_brexitpoll_1

I've always found this standard display inadequate. The story here is the gap in the two bar lengths for the Brexit Party. A secondary story is that the support for the Brexit Party might come from voters breaking from Labour. In other words, we really want the reader to see:

Redo_junkcharts_brexitpoll_1b

Switching to a dot plot helps bring attention to the gaps:

Redo_junkcharts_brexitpoll_2

Now, putting the house in order:

Redo_junkcharts_brexitpoll_2b

Why do these two polls show such different results? As the reporter explained, the answer is in how the question was asked. The Ipsos-Mori is unprompted, meaning the Brexit Party was not announced to the respondent as one of the choices while the YouGov is prompted.

This last version imposes a direction on the gaps to bring out the secondary message - that the support for Brexit might be coming from voters breaking from Labour.

Redo_junkcharts_brexitpoll_2c

 

 

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