Display a correlogram or partial autocorrelation plot             (STB-7: sts2)
-----------------------------------------------------


	^ac^  varname [^, l^ags^(^#^)^ graph_options ]      (under version 5.0)
	^bl_ac^  varname [^, l^ags^(^#^)^ graph_options ]   (under version 6.0)
        
	^pac^ varname [^, l^ags^(^#^) noc^ons graph_options ]     (version 5.0)
	^bl_pac^ varname [^, l^ags^(^#^) noc^ons graph_options ]  (version 6.0)



Description
-----------

^ac^ graphs the first ^lag()^ autocorrelations along with a one-standard-error
band.  

^pac^ graphs the first lag() partial autocorrelations along with a one-standard-
error band.  


Version control
---------------

In order to handle the naming conflict between these two commands and the new
Stata 6 time-series commands ^ac^ and ^pac^, the new commands resolve the
the conflict using version control.  For example, the Becketti library commands
^ac^ and ^pac^ may be invoked as follows in Stata 6:

        . ^version 5.0^
        . ^ac  x, lags(3)^    
        . ^pac x, lags(3)^

Alternatively, you can access these commands directly using their new names:

        . ^bl_ac x, lags(3)^
        . ^bl_pac x, lags(3)^


Options
-------

^lags(^#^)^ specifies the number of lags.  If not specified, the first 20 auto-
    correlations are graphed.

^nocons^ suppresses the constant in the individual regressions; see Methods and
    Formulas, below.

graph_options refers to any of the options of ^graph, twoway^, with the exception
    of ^border^, ^connect()^, ^pen()^, ^rlabel()^, ^xlabel()^, and ^yline()^.  You may
    specify ^saving()^, ^symbol()^, and the titling options.


Examples
--------

 . ^version 5.0^
 . ^ac gnp^
 . ^ac gnp, lags(24)^
 . ^pac gnp^
 . ^pac gnp, lags(24)^


Methods and formulas
--------------------

For ^ac^, the standard error of the autocorrelations is estimated by Bartlett's
approximation.

For ^pac^, the standard error is approximated by 

                                     1
                        -----------------------------
                         sqrt(number of observations)

The partial autocorrelations are estimated from a sequence of ^lags()^ reg-
ressions.  If ^nocons^ is specified, the regressions will be estimated without
a constant.  Otherwise, a constant will be included in the regressions.



Author
------
 
Sean Becketti, Federal Reserve Bank of Kansas City.


Also see
--------

    STB:  ^sts1 (STB-5)^
          ^sts2 (STB-7)^
On-line:  help for @ac@, @pac@, @dfuller@, ^dickey^, ^lag^
 Manual:  ^[R] corrgram^
          ^[R] dfuller^