= Stata System Variables =

Stata exposes internal data through '''system variables'''. These are sometimes also referred to as ''underscore variables''.

<<TableOfContents>>

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== System Variables ==

`_N` stores the number of cases in the current data set. This is affected by commands that operate on a subset of data, such as any command gated by a [[Stata/Logic#By|by structure]].

This enables the direct computation of rates. For example:

{{{
count if error==1
assert (r(N)/_N) < .1
}}}

`_n` refers to a case's index in the current data set. This is similarly affected by commands that operate on a subset of data.

To create an 8-digit unique identifier number, try:

{{{
generate double UniqueID = 10000000 + _n
}}}

`_rc` stores the return code of the last command or program. This would commonly be accessed like:

{{{
capture assert duplicate==0
if (_rc!=0) {
  display "There are duplicates!"
}
}}}

Statistical programmers should be advised that Stata follows the POSIX model for return values: 0 indicates success, any other integer value indicates an error.

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== Model Variables ==

For the most-recent model, several system variables are stored:

 * `_b[VAR]` is the coefficent for `VAR`
 * `_coef[VAR]` is a reference to `_b[VAR]`
 * `_se[VAR]` is the standard error for `VAR`
 * `_cons` is 1 whenever accessed directly, but is variable when accessed indirected (as through `_b[_cons]`)

In the context of multiple-equation models, an additional bit of syntax is necessary to indicate the '''equation number'''. This can either be specified in brackets preceding the system variable (`[#2]_b[VAR]`), or inside the brackets preceding the variable specification (`_b[#2:VAR]`). If an equation number is not specified, `#1` is implied. In the context of a single-equation model, `#1` is the only valid reference and generally is not specified.

There are several aliases enabled by this syntax. All of the following are equivalent.

{{{
_b[VAR]
_coef[VAR]
[#1]_b[VAR]
[#1]_coef[VAR]
[#1][VAR]
_b[#1:VAR]
_coef[#1:VAR]
}}}



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