AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Use of Numerals With Units
Units of Measure

17.4.1 Expressing Quantities.

Arabic numerals are used for quantities with units of measure (see 18.1, Use of Numerals). By SI convention, it is preferable to use only numbers between 0.1 and 1000 and to use the appropriate prefix for expressing quantities. For example, 0.003 mL is expressed as 3 μL; 15 000 g is expressed as 15 kg.

Some clinical measurements are expressed in quantities and units that may have numbers outside this preferred range. For such values, the use of scientific notation is acceptable.

20 000 000 A may be expressed as 20 million amperes or as 2 × 10⁷ A

Reported SI values should follow recommendations for preserving the proper number of significant digits (see 19.4.2, Rounding). The use of these increments is intended to eliminate reporting results beyond the appropriate level of precision.

17.4.2 Decimal Format.

The decimal format is recommended for numbers used with units of measure. Fractions should not be used with SI units.

Mixed fractions occasionally are used in text to indicate less precise measurements and most commonly involve units of measure that represent time.

After more than 7½ years of investigation, the effort to develop a new vaccine was abandoned.

The decimal format also could be used:

After more than 7.5 years of investigation, the effort to develop a new vaccine was abandoned.

Numerical values less than 1 require placement of 0 before the decimal marker.

However, certain statistical values, such as α levels and P values, should be reported without the use of 0 before the decimal marker (see 18.7.1, Decimals, and 19.5, Glossary of Statistical Terms).

The sample size was based on detecting a 10% difference in the primary outcome measure, using a 2-sided α level of .05.

Statistical significance was defined as P  <  .001.

17.4.3 Number Spacing.

By SI convention, the decimal point is the only punctuation mark permitted in numerals, and it is used to separate the integer and decimal parts of the number. The SI does not use commas in numbers in particular because the comma is used in some countries as the decimal sign. Integers (whole numbers) with more than 4 digits are separated into groups of 3 (using a thin space, a space character that is usually 1⁄5 or 1⁄6 the width of an em dash; to insert a thin space using Unicode in Microsoft Word, type the code 2009 where the thin space should appear, set the cursor after “9,” and press Alt + X on the keyboard) with respect to the decimal point. Four-digit integers are closed up (without a space). Decimal digits also are grouped in sets of 3 digits beginning at the decimal sign, with the same closed-up spacing for 4-digit groups (see 18.1.1, Numbers of 4 or More Digits to Either Side of the Decimal Point).

However, certain types of numerals that have more than 4 digits are expressed without spacing, such as street addresses, postal codes, page numbers, and numerals combined with letters, including trial registration identifiers.

Chicago, IL 60610

This study was supported by grant MCH-110624.

Trial Registration: ClinicalTrials.gov identifier: NCT00381954

17.4.4 Multiplication of Numbers.

Multiplication of numbers should be indicated by the multiplication sign (×) and may be used to express area (eg, a 15 × 35-cm² burn), volume (eg, a 5.2 × 3.7 × 6.9-m³ cube), matrixes (eg, a 2 × 2 table), magnification (×30 000), or scientific notation (eg, 3.6 × 10⁹/L).

The multiplication sign in equations is set off by thin spaces on either side (see 20.9, Spacing With Mathematical Symbols). In situations such as expression of magnification (eg, original magnification ×30 000), it is set closed up to the number because this is not an equation.

17.4.5 Indexes.

An index generally refers to a quantity derived from a ratio of 2 (or more) measurable quantities and often is used to compare individuals with each other or with normal values. Except for products or quotients that represent specific derived SI units of measure (see 17.2.2, Products and Quotients of Unit Symbols), the ratio of SI units used to create indexes does not represent an SI convention.

At first mention in the text, the formula used to calculate the index should be described; thereafter, the numerical value for the index may be given without units attached to it. For figures or tables, the formula should be included in legends or in footnotes, respectively. However, the formula used to calculate an index need not be included in the abstract of an article.

body mass index, calculated as weight in kilograms divided by height in meters squared (BMI of 30)

cardiac index, calculated as cardiac output in liters per minute divided by body surface area in square meters