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

Displayed vs Run-in
Mathematical Composition

Mathematical formulas and other expressions that involve special symbols, character positions, and relationships may present difficulties in clarity in print and online publications. Avoid ambiguity through proper use of parentheses and brackets and adhere to typographic conventions and capitalization rules in equations (see 8.5, Parentheses and Brackets, and 21.0, Editing, Proofreading, Tagging, and Display). Some software applications, such as MathType,¹ and document markup languages, such as LaTeX,² provide support for typesetting, formatting, and displaying complex mathematical notation and equations.

20.1 Displayed vs Run-in.

Simple formulas may remain within the text of the manuscript if they can be set on the line (ie, run in; see 20.2, Stacked vs Unstacked Fractions or Formulas):

The pulmonary vascular resistance index (PVRI) was calculated as follows: PVRI  =  (MPAP  −  PCWP)/CI, where MPAP indicates mean pulmonary artery pressure; PCWP, pulmonary capillary wedge pressure; and CI, cardiac index.

Long or complicated formulas should be set off and centered on a separate line.

For any given sample, s, we then defined the smoking index score, SI(s), as

where wc is +1 (—1) if the smoking-associated CpG, c, is hypermethylated (hypomethylated) in smokers and where βcs is the β-methylation value of the CpG c in sample s.

Whether run in to the text or set below it and centered, an equation is an element of the sentence that contains it. Punctuation and grammatical rules thus apply, just as they do for all other sentence elements. For example, if the equation is the last element in a sentence, it must be followed by a period. If there are 3 equations in a list, they must be separated by commas, and the final equation must be preceded by “and.”

If there are numerous equations in a manuscript or if equations are related to each other or are referred to after initial presentation, they should be numbered consecutively. Numbered equations should each be set on a separate line, centered, with parenthetical numbers set flush left or flush right. More complex equations can be numbered sequentially and displayed in a separate box. See examples in Box 20.1-1.

Box 20.1-1. Examples of Complex Equations

Equation 1. Full model

Equation 2. Probabilities of sexual transmission

Equation 3. Probability of IDU transmission

Equation 4. Per-act transmission risk, vaginal sex

A_j,k,m indicates annual per-partner number of episodes for sex act or role; c, proportion of transmissions averted by condom use (condom effectiveness); D_j,k,l, sex partner serodiscordance; D_u, drug partner serodiscordance; F, annual per-partner episodes of IDU distributive sharing; I_u, injection drug use (IDU); j, partner sex (1 if male, 2 if female); J, maximum number of partner sex types (1 if partner is female, 2 if male); k, partner main or casual type; l, protected or unprotected sex partner (1 if unprotected, 2 if protected); m, insertive or receptive sex role; RR₁, per-act relative risk of needle sharing compared with vaginal sex; RR_{S j,m}, per-act relative risk of act and role compared with vaginal sex; S_j,k,l,m, vaginal or anal act or role with partner; T_{j, k}, total partners of sex j and type k; tx, transmission; and U, total IDU-sharing partners.

Standard abbreviations should be used in expressing units of measure (see 13.12, Units of Measure). For short, simple equations, it may be preferable to express an equation as words in the running text rather than to set it off as an actual formula:

Attributable risk is calculated by subtracting the incidence among the nonexposed from the incidence among the exposed.

Note: Do not present a formula or equation as a unit of measure (eg, do not use kg/m² as a unit of measure for body mass index, which is calculated as weight in kilograms divided by height in meters squared, because it is the actual equation that is used to determine body mass index). Therefore, the sentence “His BMI was 30.0 kg/m²” is incorrect; instead, the sentence should read, “His BMI was 30.0.”