How to Read a Histogram

Oct 20th, 2008 | By | Category: Stats

Nate Silver, the wonky head of the mathematically rigorous election projection site, has a computer model that uses all of the available polling, weighted for accuracy, demographics and the rest, to run through ten thousand possible elections every day. Each one of these simulated elections pops out an electoral vote total for Obama.

What’s the best way to display all this data? A histogram.
Here’s Nate’s:

Along the bottom, on the horizontal, are the possible electoral vote counts for Obama.

For each one, from zero to five hundred thirty eight, on the vertical are the number of times this Obama electoral vote count happened during his ten thousand simulations. The tallest peaks are the most likely outcomes during the simulation. The low tails are things that are possible, but not very likely.

Many of the closer followers of, like the Stranger’s own Anthony Hecht, tend to focus more on the big Obama victory pie chart. Over the past few days, Obama’s number has drifted down a bit, from a peak around 96% to the low nineties today.

Look at the histogram for today:

McCain is all tail, no peak. The peaks are still strongly skewed to an Obama blowout.

The histogram tells you, in much more detail than an number or a pie chart, the chances of the different outcomes in crisp (and this case comforting) detail.

I love histograms.