Writing about numbers

  1. Numbers: Numbers should consistently have the same number of decimal places. If one result is 13.35mg and another is fifteen mg, the latter should be written 15.00mg. Numbers should always have units (mm, °C, µg, etc.) if they express quantities. Scientific journals usually require that SI units be used.

  2. Symbols: In English, symbols should follow, with a space, the numbers with which they are associated. Thus 10.8 %, not 10.8%. Currency symbols ($, €, £) are usually placed immediately before the associated number. All units should be in accordance with the SI.

  3. Ratios, proportions and rates are words with precise scientific meanings. A ratio is the result of dividing one quantity by another. The numerator and the denominator are two separate and distinct quantities. Neither includes the other; often they are measured in the same units, but this is not essential. Thus

sex ratio =  no. of males : no. of females.

  1. A proportion is a ratio in which the numerator is included in the denominator. Thus

male proportion = no. of males / (no. of males + no. of females)

  1. Ratios and proportions are summary phenomena. They measure status rather than events or changes, and they have no units.

  2. A rate is a measure of change in one quantity (y) per unit of another quantity (x) on which y depends. Speed is a rate, measured in distance per unit of time, hence metres per second or kilometres per hour. Prevalence is a proportion, so there are no units; incidence is a rate, expressed in new cases per unit time. (See Elandt-Johnson RC: Am J Epidemiol 1975;102:267-71.) NB dentists: DMFT is not a rate!

  3. Write Dates in DMY (day, month, year) format. When referring to decades, do not use an apostrophe (e.g. the 1990s, not the 1990’s).