Fighting Statistical Illiteracy

Inscrutable Bar Graph

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The UK Smart Centre has some great bar graphs involving alcohol usage by age and sex for kids 11-15.  Press Start and select any of the five right tabs: Week, Freq, Mean, Type and Units. 

 But the percentage bar graph for the Lesson tab is inscrutable — IMHO :-) 


One source of confusion is the cryptic “Lessons: Yes/No” heading at the bottom.  Based on a related paper, Alcohol and a Mash-up: Assessing Student Understanding, it appears that some students had lessons on problems related to drinking (Yes) and some did not (No).

Since there isn’t much difference between the left and right graphs, it looks like the lessons didn’t have much effect on students’ drinking behaviors.

 But the big source of confusion is the percentages. What are they percentages of?   Notice that the “Never” group (the brown-colored bar on the right in each series) is always 100%.  It seems that this Never group is the basis for comparison and all other groups are shown as a ratio or percentage of the Never group.  With some effort we can figure out that about 30 percent of 15 year old boys who had lessons never drink.   Can you see how?

 But the real question is “Why bother?”  Is there a reason this graph is so inscrutable?  Or is there some way to read this percentage bar chart so it really does make good sense?

Written by schield

November 30th, 2009 at 11:10 pm

Posted in 2Assembly,Ratios

Tagged with ,