{smcl}
{* 25jun2006}{...}
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help for {hi:roblpr}    (SSC distribution 25 Jun 2006)
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{title:Estimate long memory in a set of timeseries via Log-Periodogram Regression}

{p 8 17}{cmd:roblpr}
{it:varname}
[{cmd:if} {it:exp}]
[{cmd:in} {it:range}] 
[{cmd:,} {cmdab:powers(}{it:numlist}{cmd:)}
{cmdab:l}(#) {cmdab:j}(#) {cmdab:constraints(}{it:numlist}{cmd:)}]

{p 4 4}{cmd:roblpr} is for use with time-series data. 
You must {cmd:tsset} your data before using {cmd:roblpr}; see help {cmd:tsset}. 
{cmd:roblpr} supports the {cmd:by} prefix, which may be used to operate on each 
time series in a panel. Alternatively, the {cmd:if} qualifier may be used to specify 
a single time series in a panel.


{title:Description}

{p 4 4}{cmd:roblpr} computes the Robinson (1995) multivariate semiparametric
estimate of the long memory (fractional integration) parameters, {it:d(g)}, of a
set of timeseries, y(g), g=1,G. G may be one. If a series
exhibits long memory, it is neither stationary (I[0]) nor is it a unit 
root (I[1]) process; it is an I({it:d}) process, with {it:d} a real number. When
applied to a set of timeseries, the {it:d(g)} parameter for each series is 
estimated from a single log-periodogram regression which allows the
intercept and slope to differ for each series. The standard errors for
the estimated parameters are derived from a pooled estimate of the variance
in the multivariate case, so that their interval estimates differ from those
of their univariate counterparts.{p_end}

{p 4 4}
A choice must be made of the number of harmonic ordinates to be included
in the spectral regression. One of the innovations of Robinson's estimator
is that it is not restricted to using a small fraction of the ordinates of
the empirical periodogram of the series. The estimator also allows for the
removal of one or more initial ordinates, and for the averaging of the
periodogram over adjacent frequencies. {p_end}

{p 4 4}
Results of the estimation are returned after the command; type {cmd:return list}
to see the returned results. If more than one series is specified in
varlist, an F-test of the equality of the d(g) coefficients is performed,
and those values are returned as well.{p_end}


{title:Options}

{cmd:powers(}{it:numlist}{cmd:)} specifies the number of ordinates ({it:nord}) entering the 
    log-periodogram regression. Each value in numlist is taken as the value {it:p}
    in {it:nord = N^p}.  The default value of {it:p}, following Robinson, is 0.9. More
    than one {it:p} may be specified in numlist only if there is a single
    variable in the {it:varlist}.

{cmd:l}(#) specifies the number of initial ordinates to be excluded from the 
    log-periodogram regression. Some researchers have found that such
    exclusion improves the properties of tests based on log-periodogram
    regressions. The default value of {it:l} is zero.

{cmd:j}(#) specifies that the log-periodogram is to be computed from sums of 
    adjacent periodogram ordinates. The default value of {it:j} is 1, so that no
    averaging is performed. With {it:j}=2, the number of ordinates is halved; with
    {it:j}=3, divided by three; and so on.

{cmd:powers(}{it:constraints}{cmd:)} specifies the constraint numbers of the linear constraints
    to be applied during estimation; see {cmd:help constraint}.  The default is to
    perform unconstrained estimation.

    This option allows the imposition of linear constraints prior to
    estimation of the pooled coefficient vector. For instance, if the
    varlist contains prices, dividends, and returns, and your prior (or
    previous findings) states that prices' and dividends' order of integration
    is indistinguishable, one might impose that constraint to improve the
    power of the F-test provided by {cmd:roblpr}.  One would specify the
    constraints prior to the {cmd:roblpr} command, and then provide the list of
    constraints in the constraints option to {cmd:roblpr}.
    
    Technical note on constraints.  When constraints are imposed it is
    difficult to identify the number of numerator degrees of freedom in the 
    test for equality of d coefficients reported at the bottom of {cmd:roblpr}'s 
    output. Since constraints can be of any general form and it is possible
    specify constraints that are not unique, {cmd:roblpr} determines the
    degrees of freedom from the rank of the matrix used to compute the Wald
    statistic. Determining that matrix rank from a numerical standpoint can be
    problematic, in which case {cmd:roblpr} may overstate the number
    of constraints being tested and thereby incorrectly compute the numerator 
    degrees of freedom for the test.  This rarely has a meaningful impact on
    the statistical test, but you may wish to test only the unconstrained
    coefficients if the computed degrees of freedom are wrong.

    For example, after the final example below, we could perform the test by
    typing {cmd:test ftap == ftaret}.  In this case, the degrees of freedom were
    correct, so we needn't have gone to the trouble.


{title:Examples}

{p 4 8}{stata "use http://fmwww.bc.edu/ec-p/data/Mills2d/fta.dta": . use http://fmwww.bc.edu/ec-p/data/Mills2d/fta.dta}{p_end}

{p 4 8}{stata "roblpr ftap":. roblpr ftap}{p_end}

{p 4 8}{stata "roblpr ftap ftadiv":. roblpr ftap ftadiv}{p_end}

{p 4 8}{stata "roblpr ftap ftadiv ftaret, j(2)":. roblpr ftap ftadiv ftaret, j(2)}{p_end}

{p 4 8}{stata "roblpr ftap, power(0.7 0.75:0.9)":. roblpr ftap, power(0.7 0.75:0.9)}{p_end}

{p 4 8}{stata "roblpr ftap, j(2)":. roblpr ftap, j(2)}{p_end}

{p 4 8}{stata "roblpr D.ftap, j(2) l(1)":. roblpr D.ftap, j(2) l(1)}{p_end}

{p 4 8}{stata "constraint define 1 ftap = ftadiv":. constraint define 1 ftap = ftadiv}{p_end}
{p 4 8}{stata "roblpr ftap ftadiv ftaret, c(1)":. roblpr ftap ftadiv ftaret, c(1)}{p_end}


{title:Authors}

{p 4 4}Christopher F. Baum, Boston College, USA{break} 
       baum@bc.edu
       
{p 4 4}Vince Wiggins, StataCorp LP{break} 
        vwiggins@stata.com
        
{title:References}
{p}Robinson, P.M. Log-Periodogram Regression of Time Series with Long
       Range Dependence. Annals of Statistics 23:3, 1995, 1048-1072.{p_end}
      

{title:Also see}

{p 4 13}On-line: {help regress}, {help time}, {help tsset}, {help ac}, 
{help corrgram}; {help gphudak} (if installed), {help modlpr} (if installed)
{p_end}
