Mathematicians and scientists use lots of other specific Greek and Roman letters as symbols for

specific constants, but you need only a few of them in your biostatistics work. π and e are the most

common, and we define others in this book as they come up in topics we present.

Variables

The term variable has two slightly different meanings:

In mathematics and engineering, a variable is a symbol that represents some quantity in a

formula. It is usually a letter of the alphabet. You are probably used to seeing variables like x and y

in algebra, for example.

In statistics and computer science, a variable is a name referring to a single data value or an

entire field, which is a column of data in a spreadsheet or database. The variable name is made up

of letters (like SBP for systolic blood pressure), but may also contain numbers (such as SBP1,

SBP2, and SBP3). Technically, the variable name refers to a place in the computer’s memory

where the data value or field is stored. For example, a computer programmer writing a statistical

software program may ask if the variable SBP is greater than or equal to 120 mmHg.

The names of variables may be written in uppercase or lowercase letters depending upon typographic

conventions or preferences, or on the requirements of the software being used.

Variables are always italicized in typeset formulas, but not in plain text formulas.

Focusing on Operations Found in Formulas

A formula tells you how the building blocks of numbers, constants, and variables are to be combined.

In other words, a formula is a recipe for the calculations you’re supposed to carry out on these

quantities. But formulas are not always easy to read. A particular symbol — such as the minus sign —

can be interpreted differently, depending upon the context of the formula. Also, a particular

mathematical operation like multiplication can be represented in different ways in a formula. In the

following sections we explain the basic mathematical operations you see in formulas throughout this

book and describe two types of equations you’ll encounter in statistical books and articles.

Basic mathematical operations

The four basic mathematical operations are addition, subtraction, multiplication, and division (ah, yes

— the basics you learned in elementary school). Different symbols are associated with these

operations, as you discover in the following sections.

Addition and subtraction

Addition and subtraction are always indicated by the + and – symbols, respectively, placed between

two numbers or variables. Compared to the plus sign, the minus sign can be tricky when it comes to

interpreting it in a formula.

A minus sign placed immediately before a number indicates a negative quantity. For example, –

5° indicates five degrees below 0, and –5 kg indicates a weight loss of 5 kilograms.