I found this today in an article linked by John Walkenbach on Google Plus, which displays results from a survey about air travel:-
The first thing you’re likely to notice other than 3d pie hell is that the percentages don’t add up. At all. There is an explanation from this however – respondents were allowed to choose more than one answer.
So why use three pie charts when the data isn’t being displayed as parts of a whole in the first place??
The first pie totals 159%, the second 37% and the third 11%. That’s a total of 207% over three charts. What?
I don’t know about you, but this completely and utterly confuses me. Surely a better method would be just to show the total numbers of each answer chosen, or to in some way rank them in order of importance at the point of selection. The three disjointed pie charts do absolutely nothing for the data as far as I’m concerned.