Single equal: Set equal

The single equal, =, is used as a set equal operator. Old time
Pascal programmers will recognize it as, :=.
It is used in
the generate, replace and recode commands. Examples:

generate tot = read + write + math
replace mean = total/freq
recode x 5=1 4=2 2=4 1=5

The single equal is also used in some of the multivariate commands, such as, mvreg and manova.
For example,

mvreg read write math science = gpa gender honors
manova read write math science = gender program gender*program

Double equal: Test for equality

The double equals, ==, is used to test for equality.  It is sometimes called logical equals because
it is part of a logical test that returns either a one (true) or a zero (false). Here are
some examples:

regress write read female if ses==2
list read write math if prog==1
assert gender==1 

The double equals can also be used in an if command in a do-file or ado-file.
For example,

if stage==2 {
       local s2 "stage 2 analysis"
       display `s2'
       regress yvar `xvar1' `xvar2'
}

You can create some interesting commands that have both a single equal and double equal
signs.  For
example,

generate hises = ses==2

This command creates a zero/one variable, hises. The expression ses==2 has the
value of one when ses (logically) equals two and has the value zero otherwise. You
need to take care when using this type of command when there are missing values. The missing values
will get set to zero which may or may not be what you want.