help pwmc
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Title

     pwmc -- Pairwise multiple comparisons (unequal variances)


Syntax

     Syntax for pwmc

        pwmc varname , over(varname) [options]


     Immediate command

        pwmci stats1 stats2 stats3 [ ... statsk ] [ , options ]


     where stats is

        #obs #mean #sd


Description

    pwmc performs pairwise comparisons of means. It computes pairwise
    differences of the means of varname over the levels of over(varname).
    Confidence intervals are derived using procedures allowing for unequal
    variances across groups.

    pwmc implements Dunnett's C, GH and T2 procedures, as proposed by Dunnett
    (1980), Games and Howell (1976) and Tamhane (1979).

    pwmci is the immediate form of pwmc. See immed.


Options

    over(varname) is required and specifies that means are computed for each
        level of varname.

    procedure(proc) specifies the procedure to be used to derive confidence
        intervals. proc may be any combination of C, GH and T2 and case does
        not matter. Default is to report confidence intervals using each of
        the procedures.

    level(#) specifies the confidence level, as a percentage, for confindence
        intervals. The default is level(95).  See set level.

    varlabel displays variable labels instead of variable names.

    vallabel displays value labels instead of numeric codes.

    cformat(%fmt) specifies how to format means, standard errors, and
        confidence limits.


Example

        . pwmc wage , over(race)

        . pwmci 1637 8.08 5.96 583 6.84 5.08 26 8.55 5.21


Saved results

    pwmc saves the following in r():

    Scalars
      r(k)           number of levels of over(varname)
      r(ks)          number of differences

    Macros
      r(depvar)      varname from which means are computed
      r(over)        varname in over(varname)

    Matrices
      r(diff)        differences of means
      r(Var)         variances of differences
      r(nuhat)       degrees of freedom
      r(ll_proc)     lower limit of confidence interval using procedure proc
      r(ul_proc)     upper limit of confidence interval using procedure proc
      r(A_proc)      critical values using procedure proc (cf. Dunnett, 1980)


References

    Dunnett, Charles W. (1980). Pairwise Multiple Comparisons in the Unequal
    Variance Case, Journal of the American Statistical Association, 75(372),
    796-800.

    Games, Paul A., Howell, John F. (1976). Pairwise Multiple Comparison
    Procedures with Unequal N's and/or Variances: A Monte Carlo study,
    Journal of Educational Statistics, 1(2), 113-125.

    Tamhane, Ajit C. (1979). A Comparison of Procedures for Multiple
    Comparisons of Means with Unequal Variances, Journal of the American
    Statistical Association, 74(366), 471-480.


Acknowledgments

    Matthew K. Lau's 'DTK' package for R was helpful developing pwmc.


Author

    Daniel Klein, University of Kassel, klein.daniel.81@gmail.com


Also see

    Online: pwmean, oneway

    if installed: dunnett, prcomp