Hi Julia,

I agree with Ted that you can treat this as a 3 group design, but the
special nature of the groups would suggest some non default hypotheses.
 Suppose you just have a pre and a post test for each group.  Where D1 =
low dose, D2 = high dose, C = normal, of interest could be:

(D1pre & D2pre) vs. Cpre  (are the D deficient groups lower than normal at
baseline)
D2post vs. Cpost   (is the high dose group diffferent from normal after
treatment?)
D1post vs. Cpost   (is the low dose group different from normal after
treatment?)
D2post vs. D1post  (are there differences between high and low dose after
treatment?)

You could put a more specific hypothesis on some of these, like:

(D1pre = D2pre) < Cpre  (pretreatment D groups are equal and less than C)

Cpost > D2post > D1post  (post treatment normal are still highest, followed
by high dose, followed by low dose)

If you define change scores (or if you have multiple assessments,
trajectories), say: dD1, dD2, dC

dD2 > dD1 > dC  (high dose group will change more than low dose group will
change more than normal)

Cheers,

Josh




On Sat, Oct 12, 2013 at 5:50 AM, Ted Wright <cewright@uci.edu> wrote:

> You should treat this as a 3-group design. There is no nesting or crossing,
> but you can still compare the outcomes for the three groups.
>
> Ted Wright
>
> -----Original Message-----
> From: r-sig-mixed-models-bounces@r-project.org
> [mailto:r-sig-mixed-models-bounces@r-project.org] On Behalf Of Yla Savh
> Sent: Saturday, October 12, 2013 4:11 AM
> To: r-sig-mixed-models@r-project.org
> Subject: [R-sig-ME] Issue with experimental design?
>
> Dear forum members, Could you please help me confirm if there is a problem
> with the experimental design? There are two initial groups (vitamin D
> deficient and normal group). Then, the D deficient group has two treatments
> (with a low D dosage and a high D dosage), while the normal group has only
> one treatment (maintenance therapy).
> Initially, I thought it might be a nested design (treatment nested within
> groups, where the treatment is a fixed effect, and groups and treatment
> nested in groups are random effects. However, I do not think it is a
> correct
> design as the groups did not include the same treatments. Am I correct?
> I see only one solution where we will have only one or two groups and the
> same treatments should apply to each of them. Are there other
> solutions?Thanks,Julia
>
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>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://joshuawiley.com/
Senior Analyst - Elkhart Group Ltd.
http://elkhartgroup.com

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