STATISTICA Basic Program: CfdDiff.stb

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STATISTICA BASIC Program CfdDiff.stb

{ This program will compute confidence intervals for the differences of group means on one or more dependent variables from two independent samples (Milliken & Johnson, 1984). This program uses the data from the Grouping Scrollsheet in the Basic Statistics and Tables module. To use this program, create the Grouping Scrollsheet for a specific between-group factor with two levels and a set of dependent variables in t-test for independent samples of the Basic Statistics and Tables module. Click on T-tests to obtain the Grouping Scrollsheet so that it is the active Scrollsheet. The program will perform the computations and add three additional columns to the Scrollsheet. The columns contain the differences of the group means, the upper confidence limits and the lower confidence limits of the differences of the means. Users will be prompted to specify the alpha value for the desired confidence intervals.

Program written, modified, or edited at StatSoft, Inc.}

RandomAccess;

NoDataFileVariableNames;

{Make sure the users are ready to compute the confidence intervals.}

     Startmsg := DisplayMessageBox(MB_OKCANCEL, 'Confidence Intervals of the difference of
two means', 'This program will compute the confidence intervals of the difference of two
means for independent samples. The Grouping Scrollsheet needs to be active on the screen.
You will be prompted to input the alpha value. If you are ready, click "OK".');

     If Startmsg = IDCANCEL then stop;

{Start the procedure.}
{Read the Scrollsheet created from t-test independent samples in the Basic Statistics and Tables
module into a matrix.}
     scrollsheet := GetScrollsheet (0);
     ncols := ScrollsheetGetNbCols(scrollsheet);
     nrows := ScrollsheetGetNbRows(scrollsheet);
     ReDim matrix(nrows,6), tempmtx(nrows,1), dffmean(nrows), poolsd(nrows), tvalue(nrows),
Ucfd(nrows), Lcfd(nrows);
     ScrollsheetGetMatrix(scrollsheet,1,1,matrix);
{Read alpha value from users input.}
     DisplayNumericInputBox('Significance Level for Confidence Interval', 'Alpha value', alpha);
     If alpha = 0 then Stop;
{Get the right columns for computation.}
     ncoln := 2;
     For i := 3 to ncols do
      begin
             ScrollsheetGetColName (scrollsheet, i, Name1$, Name2$);
              If (Name1$ = 'Valid N') then
          begin
            ncoln := ncoln + 1;
            ScrollsheetGetMatrix(scrollsheet,1,i,tempmtx);
            MatrixCopy(tempmtx, 1,1,nrows,1,matrix,1,ncoln);
          end;
              If (Name1$ = 'Std.Dev.') then
          begin
            ncoln := ncoln + 1;
            ScrollsheetGetMatrix(scrollsheet,1,i,tempmtx);
            MatrixCopy(tempmtx, 1,1,nrows,1,matrix,1,ncoln);
          end;
         end;
{Start the computation.}
     For i :=1 to nrows do
     begin
{Compute the difference of two group means.}
         dffmean(i) := matrix(i,1)-matrix(i,2);
{Compute the pooled standard deviation.}
         pooldf := matrix(i,3)+matrix(i,4)-2;
         poolsd(i) :=sqrt( ((matrix(i,3)-1)*matrix(i,5)^2+(matrix(i,4)-1)*matrix(i,6)^2)/pooldf);
{Obtain the t-value and compute confidence intervals.}
         tvalue(i) := VStudent(1-alpha/2, pooldf);
         Ucfd(i) := dffmean(i)+tvalue(i)*poolsd(i)*sqrt((1/matrix(i,3))+(1/matrix(i,4)));
         Lcfd(i) := dffmean(i)-tvalue(i)*poolsd(i)*sqrt((1/matrix(i,3))+(1/matrix(i,4)));
     end;

{Arrange the output.}
{Create appropriate new column names.}
     lower$ := "-" + Str((1-alpha)*100,6,3) + "%";
     upper$ := "+" + Str((1-alpha)*100,6,3) + "%";
     mean1$ := "Mean 1 ";
     mean2$ :="-" + " Mean 2";
     ScrollsheetAppendColumns(scrollsheet,3);
            ScrollsheetSetColumnName(scrollsheet,ncols+1,mean1$,mean2$);
     ScrollsheetSetColumnName(scrollsheet,ncols+2,"Confid.", lower$);
     ScrollsheetSetColumnName(scrollsheet,ncols+3,"Confid.", upper$);
{Define the column format.}
     For i := 1 to 3 do
     ScrollsheetSetColumnFormat(scrollsheet, ncols+i,SCF_OPTIMAL, ?FmtParam);
{Enter the results to the scrollsheet.}
     ScrollsheetSetMatrix(scrollsheet,1,ncols+1,dffmean);
     ScrollsheetSetMatrix(scrollsheet,1,ncols+2,Lcfd);
            ScrollsheetSetMatrix(scrollsheet,1,ncols+3,Ucfd);

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