- alpha is likelihood of making a type I error (usually = 0.05)
- etasq is partial eta-squared/100 so, for example, 5.9% = 0.059
- num(erator) is product over factors comprising term of interest of (numer of levels of factor -1)
d1 is sum over the factors of (number of levels of between subject factor - 1)
d2 is product over the factors of (number of levels of within subject factor - 1)
- ntot is the total sample size
Power can also be computed using a [attachment:aov.xls spreadsheet.]
[ COPY AND PASTE THE BOXED BELOW SYNTAX BELOW INTO A SPSS SYNTAX WINDOW AND RUN; ADJUST INPUT DATA AS REQUIRED]
DATA LIST free /alpha etasq num d1 d2 ntot. BEGIN DATA. .05 0.02 2 3 2 58 .05 0.12 2 3 2 58 END DATA. set errors=none. matrix. get m /variables=alpha etasq num d1 d2 ntot /missing=omit. compute alpha=make(1,1,0). compute etasq=make(1,1,0). compute num=make(1,1,0). compute d1=make(1,1,0). compute d2=make(1,1,0). compute ntot=make(1,1,0). compute alpha=m(:,1). compute etasq=m(:,2). compute num=m(:,3). compute d1=m(:,4). compute d2=m(:,5). compute ntot=m(:,6). end matrix. compute denom=(ntot-d1-1)*d2. COMPUTE power = 1 - NCDF.F(IDF.F(1-ALPHA,num,denom),num,denom,denom*etasq/(1-etasq)). EXE. formats alpha (f5.2) num (f5.2) denom (f5.2) etasq (f5.2) power (f5.2). variable labels ntot 'Total Sample' /alpha 'Alpha' /num 'Numerator F' /denom 'Denominator F' /etasq 'Partial Eta-Sq' /power 'Power'. report format=list automatic align(center) /variables=ntot alpha num denom etasq power /title "ANOVA power (any anova)" .