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← Revision 6 as of 2025-10-24 16:25:26 ⇥
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| == System Variables == | == Metadata == |
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| `_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]]. | `_N` stores the number of cases. Ordinarily this evaluates to the number of cases in the active dataset. If the dataset is being subset or split, it evaluates to the number included in a given execution context. |
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| This enables the direct computation of rates. For example: | `_n` stores a case's index. Ordinarily this evaluates to a different number for every case, ranging from 1 to `_N`. But again, if the dataset is being subset or split, it evaluates to a case's index within the given execution context. |
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| {{{ count if error==1 assert (r(N)/_N) < .1 }}} |
---- |
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| `_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. | |
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| To create an 8-digit unique identifier number, try: | |
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| {{{ generate double UniqueID = 10000000 + _n }}} |
== Return Code == |
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| `_rc` stores the return code of the last command or program. This would commonly be accessed like: | In Unix fashion, `_rc` stores the return code of the last command or program. 0 indicates success, any other integer value indicates an error. Try: |
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| 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 == | == Models == |
Stata System Variables
Stata exposes internal data through system variables. These are sometimes also referred to as underscore variables.
Contents
Metadata
_N stores the number of cases. Ordinarily this evaluates to the number of cases in the active dataset. If the dataset is being subset or split, it evaluates to the number included in a given execution context.
_n stores a case's index. Ordinarily this evaluates to a different number for every case, ranging from 1 to _N. But again, if the dataset is being subset or split, it evaluates to a case's index within the given execution context.
Return Code
In Unix fashion, _rc stores the return code of the last command or program. 0 indicates success, any other integer value indicates an error. Try:
capture assert duplicate==0
if (_rc!=0) {
display "There are duplicates!"
}
Models
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]