Bad Nat Stat

Thomas Lumley at Stats Chat blogs on a National Facebook ad:

He points out:

To start with, the red bar is wider than the blue bar, which is a well-known graphical exaggeration technique. But that’s not the real problem. The real problem is the heights.

On my laptop screen, I measured the red bar as 61% higher than the blue bar. But $2.23/L is only 16% higher than $1.91/L: that’s nearly a four-fold exaggeration of the difference

The dark red ‘Tax’ section of the red bar is 92% higher than the dark blue ‘Tax’ section of the blue bar, but $1.12 is only 29% higher than $0.87: more than a three-fold exaggeration.

Such graphical exaggerations shouldn’t be used. They are misleading. There is a good enough story to tell on fuel prices and fuel tax without exaggerating it.

But wait, there’s more!  The blue bar is averaged over nine years of National government; the red bar is from last week.  That means the difference between the blue and red bars is partly inflation.

Also you shouldn’t compare an average over nine years with a point in time. The proper comparison would be the point in time when National left office with the point in time today – at least for the tax component. Arguably the price component could be averaged.

Overall poor judgement from National in authorising the advertisement. They shouldn’t have done it.