Introduction to Stata 8 - (GRIPS
Introduction to Stata 8 - (GRIPS
Introduction to Stata 8 - (GRIPS
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
6. Variables<br />
A <strong>Stata</strong> data set is rectangular; here is one with five observations and four variables:<br />
Variables<br />
obsno age height weight<br />
Observations<br />
1 27 178 74<br />
2 54 166 67<br />
3 63 173 85<br />
4 36 182 81<br />
5 57 165 90<br />
6.1. Variable names<br />
Variable names can be 1-32 characters, but <strong>Stata</strong> often abbreviates long variable names in<br />
output, so I recommend <strong>to</strong> use only 8 characters. The letters a-z (but not æøå), the numbers<br />
0-9 and _ (underscore) are valid characters. Names must start with a letter (or an<br />
underscore, but this is discouraged because many <strong>Stata</strong>-generated variables start with an<br />
underscore). These are valid variable names:<br />
a q17 q_17 pregnant sex<br />
<strong>Stata</strong> is case-sensitive<br />
Variable names may include lowercase and uppercase letters, but <strong>Stata</strong> is case-sensitive: sex<br />
and Sex are two different variable names. Throughout this booklet I use lowercase variable<br />
names; anything else would be confusing.<br />
6.2. Numeric variables [U] 15.2<br />
Most often you don't need worry about numeric types, but if you encounter memory<br />
problems, you should know this (see section 15.1 on Memory considerations):<br />
Type Bytes Precision (digits) Range (approx.)<br />
Integer byte 1 2 ±100<br />
int 2 4 ±32,000<br />
long 4 9 ±2×10 9<br />
Floating point float 4 7 ±10 36<br />
double 8 16 ±10 308<br />
compress can reduce the size of your dataset considerably by finding the most economical<br />
way of s<strong>to</strong>rage.<br />
11