How do I perform a non-standard comparison of means in a repeated measures anova in SPSS ?
Suppse we have five measurements at different time points measured on the same person. We wish to compare the average of the first three time point measurements with the average of the last two.
This can be done in SPSS using the below syntax. It is a bit cumbersome in that you need to specify a 5 x 5 matrix of contrasts using the special option. The contrasts need to be orthogonal so the sum of their products should equal zero. One tip is to use a row of '1s' for the first contrast. The actual contrast of interest is the second contrast (-0.333,-0.333,-0.333,0.5,0.5).
MANOVA A B C D E /WSFACTORS = TIME(5) /CONTRAST (TIME)= SPECIAL(1 1 1 1 1 -0.333 -0.3333 -0.333 0.5 0.5 1 -1 0 0 0 1 0-1 0 0 0 0 0 1 -1) /PRINT = TRANSFORM PARAMETERS(ESTIM) /WSDESIGN.
Contrast T2 tests whether the difference between the first three means and the last two means equals zero.
- Estimates for T2 --- Individual univariate .9500 confidence intervals TIME
- Parameter Coeff. Std. Err. t-Value Sig. t Lower -95% CL- Upper
- 1 -.45323949 .43002 -1.05400 .31448 -1.39970 .49322
- Parameter Coeff. Std. Err. t-Value Sig. t Lower -95% CL- Upper
Note that SPSS orthonormalises the contrast coefficients which simply signifies the sum of the squares of the contrast coefficients is one. The contrast coefficients are outputted using the /PRINT=TRANSFORM subcommand and are (-0.365,-0.365,-0.365,0.548,0.548).
Multiplying each of the elements in this contrast vector by the five time means gives -0.365(4.1666 + 4.25 + 4.583) + 0.548(4.333+3.5) = 0.45 = the outputted contrast coefficient above.