How to help people understand "...per 100,000"?

Question

It's well known that members of the public have difficulty comprehending numbers over a certain size. A common tricky issue for people communicating demographic trends is helping people make sense of rates out of 100,000. This is an important issue in helping people make sense of changing health and crime statistics, for example.

For those of you who do statistical communication for the public, what frames of reference or other analogies and aids have you found to be successful in helping people to put rates out of 100,000 in perspective? There's another question with a discussion alluding to 100,000 people being a small to medium sized city, and it seems there's disagreement over whether this analogy is helpful or not.

Answers citing evidence would be great, personal experience and ideas are helpful too.

(p.s. I'm also starting a discussion on meta on whether to tag/allow/encourage stats communications questions like this on this site)

Answer 1

In my limited experience, it's important to make comparisons concrete. Rather than saying "a small to medium sized city", say "Cambridge, Massachusetts" or "Brussels". This will likely require localization to your audience; most people aren't familiar with many cities of this population level other than those which are near them. In this respect, it can be easier to quote numbers per million. Paradoxically, this can be an easier number for people to visualize, because they have more examples of cities of the appropriate size.

Wikipedia's various lists of cities and countries by population can be very useful here. Similar techniques also work for talking about land areas, etc.