An on-line power calculator [[http://stat.ubc.ca/~rollin/stats/ssize/b1.html|is available.]] Computes total sample size needed for testing prop=const where prop and const are observed and expected (constant) proportions respectively and alpha is the (two-tailed) type I error for a given power. A calculator is available in a [[attachment:propn.xls|spreadsheet.]] [COPY AND PASTE THE SYNTAX BELOW INTO A SPSS SYNTAX WINDOW AND RUN; SET DATA AS DESIRED] {{{ DATA LIST free /prop const alpha power. BEGIN DATA. 0.45 0.5 .05 .85 0.55 0.5 .05 .85 END DATA. set errors=none. define propn (!pos !tokens(1) / !pos !tokens(1) / !pos !tokens(1) / !pos !tokens(1)). COMPUTE #POW = !4. compute #conf = 1-!3. compute #lc3 = 1. compute #ind=0. compute #chisq = 3. comment COMPUTE #LC1 = 2.000. COMPUTE #chisq1 = ((!1-!2)**2)/(!2*(1-!2)). COMPUTE #CUMF2 = 1 - NCDF.CHISQ(IDF.CHISQ(#conf,1),1,#CHISQ). compute #diff=1. SET MXLOOPS=40000. LOOP IF (#DIFF GT .00005) . + DO IF (#CUMF2 LT #pow) . + COMPUTE #LC3 = #CHISQ. + COMPUTE #CHISQ = (#chisq + 0.001). + COMPUTE #CUMF2 = 1 - NCDF.CHISQ(IDF.CHISQ(#conf,1),1,#CHISQ). + ELSE . + COMPUTE #LC1 = #chisq . + COMPUTE #chisq = (#chisq + #LC3)/2 . + COMPUTE #CUMF2 = 1 - NCDF.CHISQ(IDF.CHISQ(#conf,1),1,#CHISQ). + END IF . + COMPUTE #DIFF = ABS(#CUMF2 - #pow) . END LOOP . compute ntot = #chisq/#chisq1. if (ntot-trunc(ntot) gt 0.5) #ind=1. if (#ind eq 0) ntot=trunc(ntot)+1. if (#ind eq 1) ntot=rnd(ntot). EXECUTE . compute prop=!1. compute const=!2. compute alpha=!3. compute power=!4. formats ntot (f7.0) alpha (f5.2) prop (f5.2) const (f5.2) power (f5.2). variable labels ntot 'Sample Size Required' /alpha 'Alpha' /prop 'Observed Proportion' /const 'Constant' /power 'Power'. report format=list automatic align(center) /variables=ntot alpha prop const power /title "Sample Size required for a one sample binomial test" . !enddefine. matrix. get m /variables=prop const alpha power /missing=omit. compute prop=make(1,1,0). compute const=make(1,1,0). compute alpha=make(1,1,0). compute power=make(1,1,0). compute prop=m(:,1). compute const=m(:,2). compute alpha=m(:,3). compute power=m(:,4). end matrix. propn prop const alpha power. }}}