What's the difference between gen and egen in Stata 12?

Question

Is there a reason why there are two different commands to generate a new variable?

Is there a simple way to remember when to use gen and when to use egen?

Answer 1

They both create a new variable, but work with different sets of functions. You will typically use gen when you have simple transformations of other variables in your dataset like

gen newvar = oldvar1^2 * oldvar2

In my workflow, egen usually appears when I need functions that work across all observations, like in

egen max_var = max(var)

or more complex instructions

egen newvar = rowmax(oldvar1 oldvar2)

to calculate the maximum for each observation between oldvar1 and oldvar2. I don't think there is a clear logic for separating the two commands.

Answer 2

gen

generate may be abbreviated by gen or even g and can be used with the following mathematical operators and functions:

• + addition
• - subtraction
• * multiplication
• / division
• ^ power

A large number of functions is available. Here are some examples:

• abs(x) absolute value of x
• exp(x) antilog of x
• int(x) or trunc(x) truncation to integer value
• ln(x), log(x) natural logarithm of x
• round(x) rounds to the nearest integer of x
• round(x,y) x rounded in units of y (i.e., round(x,.1) rounds to one decimal place)
• sqrt(x)square root of x
• runiform() returns uniformly distributed numbers between 0 and nearly 1
• rnormal() returns numbers that follow a standard normal distribution
• rnormal(x,y) returns numbers that follow a normal distribution with a mean of x and a s.d. of y

egen

A number of more complex possibilities have been implemented in the egen command like in the following examples:

• egen nkids = anycount(pers1 pers2 pers3 pers4 pers5), value(1)
• egen v323r = rank(v323)
• egen myindex = rowmean(var15 var17 var18 var20 var23)
• egen nmiss = rowmiss(x1-x10 var15-var23)
• egen nmiss = rowtotal(x1-x10 var15-var23)
• egen incomst = std(income)
• bysort v3: egen mincome = mean(income)

Detailed usage explanations can be found at this link.