Sample size for repeated measure

[C. Fong]

Hello,
I like to determine a sample size for the two study groups for the following design:
Two groups comparing two primary measures (one continuous and onedichotomous) over 5 time points over 1 year (baseline, 3m, 6m, 9m, and12m). Cases will assumed to be dropped out over the time points. Maysomeone give me some guidance in getting the minimun required samplesize (power analysis). Thank you.

[Bruce Weaver]

Looking at the design, I expect that for the continuous outcome measure, you will perform a repeated measures  ANCOVA, with the baseline score as the covariate.  Is that correct?  (You may be able to perform something analogous for binary outcomes via GENLIN.)  

How much power do you want, and for what exactly?  I.e., do you mean for the main effect of Group?  Or for the Group x Time interaction?  Or do you mean for baseline vs one specific time point (e.g., the first or last follow-up)?  You need to give more information, I think.

HTH.

C. Fong wrote

Hello,
I like to determine a sample size for the two study groups for the
following design:
Two groups comparing two primary measures (one continuous and
onedichotomous) over 5 time points over 1 year (baseline, 3m, 6m, 9m, and12m). Cases will
assumed to be dropped out over the time points. Maysomeone give me some
guidance in getting the minimun required samplesize (power analysis). Thank
you.

[Maguin, Eugene]

Chunki Fong,

This is the article you need to look at. The message is from Bruce Weaver
and posted back in 2003 (Bruce, I didn't realize you were on the list back
then.)

Hello group,
        Just the other day, I came across an article in Behavior Research
Methods, Instruments & Computers (2001, Vol 33, 479-484) that showed how
to perform power analysis (and sample size estimation) for various
ANOVA/ANCOVA type models using Matrix Data and the MANOVA procedure. If
anyone is interested, here is a syntax file I wrote that performs the
analyses for the 3 examples given in the paper:

Here is a specific example.

* ---------------------------------------------------------- .
* Example 3:  Mixed design (between-within) ANOVA            .
* ---------------------------------------------------------- .

* Dependent variable:           Ratings of depression .
* Between-subjects factor:      Group (3 levels).
* Within-subjects factor:       Time (DV measured on 3 occasions).

* The effect of most interest is the Group x Time interaction.
* How many subjects do we need to have 80% power for this term?.

* Read in data in matrix format .

matrix data
 variables = group rowtype_ dv1 dv2 dv3
 /factor = group
 /format = lower nodiagonal.
begin data
1 mean 0.00 0.21 0.42
1 n 100 100 100
2 mean 0.00 0.21 0.42
2 n 100 100 100
3 mean 0.00 0.00 0.00
3 n 100 100 100
. sd 1.0 1.0 1.0
. corr 0.4
. corr 0.4 0.4
end data.

manova dv1 dv2 dv3 by group(1,3)/wsfactors dv(3)/method = unique
 /error = within+residual/matrix = in(*)/power t (.05) F (.05)
 /print signif (mult averf)/noprint param(estim).


Once you understand the structure you can easily modify this example for
your needs. I don't know what advice to offer for your dichotomous outcome
because the example will give a correct computation for a dichotomous DV.
There isn't anything else in spss. Perhaps there is something on the web for
repeated measures for dichotomous DVs. Perhaps others will comment.

Gene Maguin



________________________________

From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of
Chunki Fong
Sent: Wednesday, November 03, 2010 3:04 PM
To: [hidden email]
Subject: Sample size for repeated measure

>>>I like to determine a sample size for the two study groups for the
following design:
Two groups comparing two primary measures (one continuous and
onedichotomous) over 5 time points over 1 year (baseline, 3m, 6m, 9m,
and12m). Cases will assumed to be dropped out over the time points.
Maysomeone give me some guidance in getting the minimun required samplesize
(power analysis). Thank you.

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