Rates and Ratios
Use the virgule construction for rates when placed in parentheses (eg, 1/2) but never in running text. A colon is used for ratios (eg, 1:2). Rates should use the decimal format when the denominator is understood to be 100; otherwise, the denominator should be specified.
Examples:
Of all individuals exposed, children were affected at a rate of 0.05.
The infant mortality rate was 3 per 10 000 live births.
Not: The infant mortality rate was 3/10 000 live births.