Howell DC (1992, 1997, 2002) describes adjustments to uncorrected p-values based on the total number of pairwise group comparisons being performed. The macro in the box below implements some of these. This is an adaptation of a macro (rmpost.sps) first written by David Nicholls of SPSS.
[COPY AND PASTE THE BOX SYNTAX INTO A SPSS SYNTAX WINDOW; SELECT ALL AND RUN. EDIT THE INPUT DATA AS REQUIRED]
* enter a column of pvalues and this macro will * adjust for the number * in the column. The Ryan and Einot and Gabriel * methods are for pairwise * comparisons of group locations (e.g. means, * mean ranks) with a step size of abs(j - i)+ 1 * where the higher of the two means has an * overall rank of j and the lower overall * rank, i. * SPSS uses REGWQ to compute this for pairwise * comparison of group means in univariate * for between subs factors * Could be applied to p-values from ANY * procedure e.g. nonparametrics as just uses * p-value and number of comparisons * Create a dataset with all uncorrected * p-values and * step = abs(difference in ranks of group * locations) + 1. * adjust data input below as required. * The program creates a file called temp.sps * in My Documents folder which may be deleted * after running the macro * -99 in the output for Holm and Sidak * procedures indicates the pairwise comparison * is not tested and deemed nonsignificant * because the previous comparison was * nonsignificant (p=0.05, by default) * this may be changed by changing last line * in this box DATA LIST list / PVAL(f9.3) STEP (f2.0). BEGIN DATA 0.266 2 0.139 3 0.016 2 END DATA. set errors=none. set mprint=off. DEFINE PV(PVALUE=!TOKENS(1) /STEP=!TOKENS(1) /ALP=!TOKENS(1)). SORT CASES BY !PVALUE (A) . COMPUTE pos=$CASENUM. * Calculate the number of p values. RANK !PVALUE /n into N. * N contains the number of cases in the file. * make a submacro to be invoked from the syntax. DO IF $CASENUM=1. WRITE OUTFILE 'C:\Documents and Settings\user\My Documents\temp.sps' /"DEFINE !nbcases()"n"!ENDDEFINE.". END IF. EXE. INCLUDE FILE='C:\Documents and Settings\user\My Documents\temp.sps'. /* The number of cases in the file is now accessible using !nbcases */. COMPUTE bonferr=!PVALUE*!nbcases. IF (bonferr>1) bonferr=1. COMPUTE sidak=1-(1-!PVALUE)**!nbcases. COMPUTE holm=(!nbcases-pos+1)*!PVALUE. IF (LAG(holm,1)>!ALP | LAG(holm,1)=-99) holm=-99. COMPUTE downsidk=1-(1-!PVALUE)**(!nbcases-pos+1). IF (LAG(downsidk,1)>!ALP | LAG(downsidk,1)=-99) downsidk=-99. COMPUTE ryan=!PVALUE*!nbcases/!STEP. IF (ryan>1) ryan=1. COMPUTE eingab=1-(1-!PVALUE)**(!nbcases/!STEP). IF (eingab>1) eingab=1. FORMAT bonferr to downsidk ryan eingab (f7.3). VARIABLE LABELS PVALUE 'Original' /bonferr '1-step Bonferroni' /sidak '1-step Sidak' /holm 'Step-down Holm`s'/ downsidk 'Step-down Sidak' /ryan 'Ryan' /eingab 'Einot & Gabriel' /STEP 'Step'. EXECUTE. REPORT FORMAT=LIST AUTOMATIC ALIGN(CENTER) /VARIABLES=!PVALUE bonferr sidak holm downsidk !STEP ryan eingab /TITLE "Original and adjusted p-values". !ENDDEFINE. * changing the value of alp to re-specify * significance level PV PVALUE=PVAL STEP=STEP ALP=0.05.