Reading a Rate, Not a Count

Reading a rate means converting a raw count into a proportion of the group it came from, because counts follow population size before they follow anything else.

A report states that City A had 400 burglaries last year and City B had 100, and concludes City A is the more dangerous place. Run the conversion. City A holds 800,000 people, City B holds 40,000. City A's rate is 400 per 800,000, which is 50 per 100,000. City B's rate is 100 per 40,000, which is 250 per 100,000 — five times City A's. The raw counts pointed the wrong way, and they will point the wrong way in any comparison between groups of different sizes. The procedure is fixed: identify the count, identify the denominator it came from, divide, and state the result per a common base. Then check that the denominator is the right one. Burglaries per resident and burglaries per household are different rates, and a city of large families will look safer on the second. A rate is only as good as the population underneath it.