18 PART 1 Getting Started with Biostatistics

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 depend-

ing 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 dif-

ferently, depending upon the context of the formula. Also, a particular mathemat-

ical 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 encoun-

ter 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.