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st: Re: Comparisons in repeated measures anova


From   Jayne Lee <jaynelee@rocketmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: Comparisons in repeated measures anova
Date   Mon, 23 Jun 2008 13:31:30 +0000 (GMT)

I am stuck with evaluation of a comparison. I have carried out a repeated measures anova of two systems (labeled 3 & 8) over 11 time periods, and I want to see if the two systems differ at each time period. 

. anova deltae sys / sample|sys time sys*time, repeated(time)                        

Repeated variable: time
                                          Huynh-Feldt epsilon        =  0.2594
                                          Greenhouse-Geisser epsilon =  0.1914
                                          Box's conservative epsilon =  0.1000

                                            ------------ Prob > F ------------
                  Source |     df      F    Regular    H-F      G-G      Box
              -----------+----------------------------------------------------
                       time |     10    20.30   0.0000   0.0000   0.0000   0.0011
                sys*time |     10      5.27    0.0000   0.0076    0.0160   0.0446
                Residual |    100
              -----------+----------------------------------------------------
The design matrix has 48 columns and I tried to compare the systems at time 1 as follows:
. test _coef[time[1]*sys[3]] = _coef[time[1]*sys[8]]

 ( 1)  sys[3]*time[1] - sys[8]*time[1] = 0

       F(  1,   100) =   10.59
            Prob > F =    0.0016

I interpret this as the 2 systems being significantly different. The problem I have is that the means of the two systems are 0.618 and 0.642. When the data is plotted there appears to be no difference at this time and a t-test (I appreciate that it is not valid in this design) has a probability 0.668 which seems to agree with the plot.

I would be grateful if somebody could tell what I am doing wrong in this comparison and how to carry it out.
Jayne



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