_variables.hlp

是一个经济学管理应用软件 很难找的 但是经济学学生又必须用到

{smcl}
{* 06apr2005}{...}
{cmd:help _variables}
{hline}

{title:Title}

{pstd}
{hi:[U] 13.4 System variables (_variables)}


{title:Description}

{pstd}
Expressions may also contain {it:_variables} (pronounced "underscore
variables").  These are built in system variables that are created and updated
by Stata.  They are called {it:_variables} because their names all begin with
the underscore '_' character.

{pstd}
The {it:_variables} are

{phang}{cmd:[}{it:eqno}{cmd:]_b[}{it:varname}{cmd:]} (synonym:
{cmd:[}{it:eqno}{cmd:]_coef[}{it:varname}{cmd:]}) contains the value (to
machine precision) of the coefficient on {it:varname} from the most recently
fitted model (such as ANOVA, regression, Cox, logit, probit, multinomial
logit, and the like).  See
{hi:[U] 13.5 Accessing coefficients and standard errors} for a complete
description.

{phang}{cmd:_cons} is always equal to the number 1 when used directly and
refers to the intercept term when used indirectly, as in {cmd:_b[_cons]}.

{phang}{cmd:_n} contains the number of the current observation.

{phang}{cmd:_N} contains the total number of observations in the dataset.

{phang}{cmd:_pi} contains the value of pi to machine precision.

{phang}{cmd:_rc} contains the value of the return code from the most recent
{cmd:capture} command; see {helpb capture}.

{phang}{cmd:[}{it:eqno}{cmd:]_se[}{it:varname}{cmd:]} contains the value (to
machine precision) of the standard error of the coefficient on {it:varname}
from the most recently fitted model (such as ANOVA, regression, Cox, logit,
probit, multinomial logit, and the like).  See
{hi:[U] 13.5 Accessing coefficients and standard errors} for a complete
description.

{pstd}
The {cmd:_n} and {cmd:_N} variables are useful for indexing observations or
generating sequences of numbers.  {cmd:_n} can act as a running counter within
a by group and {cmd:_N} acts as the total number within each by group.

{pstd}
{cmd:_rc} is useful for programmers, particularly if you want to test a
command internal to a program without having the program terminate.


{title:Examples}

{phang}{cmd:. by famid: gen sibs = _N}{p_end}
{phang}{cmd:. by famid: gen birtho = _N - _n + 1}{p_end}
{phang}{cmd:. display _pi}{p_end}
{phang}{cmd:. display cos(_pi)}{p_end}
{phang}{cmd:. gen x = cos(_pi/4*_n)}{p_end}
{phang}{cmd:. gen price2 = price[_n-1]}

{phang}{cmd:. mvreg len disp gear_ratio = turn tru mpg wei}{p_end}
{phang}{cmd:. display [displ]_b[mpg]}

{phang}{cmd:. mlogit rep gear_ratio turn tru}{p_end}
{phang}{cmd:. display [5]_b[turn]}


{title:Also see}

{psee}
Manual:  {bf:[U] 13.4 System variables (_variables)}

{psee}
Online:  {helpb by},
{helpb generate},
{helpb replace},
{help subscripting}
{p_end}