Field Book: The Unstated Per

The unstated per is this site's name for the “per” that a number or a chart leaves out: per person, per year, per square kilometer. Many numbers are really ratios. When the bottom half of the ratio goes unsaid, a reader has to guess it, and a wrong guess can turn the conclusion around.

Why it matters

A count and a rate answer different questions. A total tells you how much there is in a place. A figure per person tells you how common something is among the people there, and a figure per square kilometer tells you how densely it is packed onto the land. Places that rank high on one can rank low on another. When the denominator changes, the conclusion often changes with it.

The same thing happens with time. A figure for one year and a figure for one decade are not comparable until both are stated per year. A chart that mixes them without saying so is comparing different things.

Where it shows up

Maps shaded by raw counts are the clearest case. A map that colors each area by a total, with no denominator, often ends up as a map of where people live. Charts do it too: an axis labeled with a bare number can hide whether each value is a total, an average or a share.

On this site, the Calibration card used for chart articles has a line for the unstated per. It says what each value is divided by, or that the values are plain counts.

How to spot it

When you meet a number, ask: per what? If the page does not say, look for the data file behind it, and check whether the values are totals, averages or rates. If two numbers are being compared, check that they share the same “per.”

The unstated per is one of three terms in this site's field book, alongside the remainder and the frozen error. How sources and data files are handled is set out in Where the numbers come from.