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Repeated measurements analysis....Which is the best SAS procedure?

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Repeated measurements analysis....Which is the best SAS procedure?

Dear SAS community,

I did an experiment in climate chambers (different temperatures) with plants infested with two different fungal species. To investigate the development and severity of disease, i sampled 15 plants from each treatment (per climate chamber/ temperature). The plants were used for destructive analysis i.e they were not replaced. The sampling was done per month for a total of four months.

The objective of the study was to investigate the influence of temperature on disease severity and growth of the plants.

Statistical question:

1. Should i perform a repeated measurement analysis on these data?

2. If Yes to question 1, is Proc mixed model the best statistical procedure for this type of analysis?

Your support will be highly appreciated.

Regards,

Peter.


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‎12-02-2014 01:04 PM
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Re: Repeated measurements analysis....Which is the best SAS procedure?

I don't see how this can be considered a true repeated measures design, as the subjects were destroyed for measurement.  However, you might wish to model the correlation over time using a spline or some other semi-parametric approach.

Choosing the right mixed model procedure can be difficult, but I would tend toward GLIMMIX, as you can do (almost) everything that MIXED does, plus many more things.  I am reasonably certain that your response variable for severity would not meet the assumptions of NID(0, sigma^2) for the residuals.  In GLIMMIX, you can work around this, depending on the endpoint definition.

Steve Denham

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‎12-02-2014 01:04 PM
Respected Advisor
Posts: 2,655

Re: Repeated measurements analysis....Which is the best SAS procedure?

I don't see how this can be considered a true repeated measures design, as the subjects were destroyed for measurement.  However, you might wish to model the correlation over time using a spline or some other semi-parametric approach.

Choosing the right mixed model procedure can be difficult, but I would tend toward GLIMMIX, as you can do (almost) everything that MIXED does, plus many more things.  I am reasonably certain that your response variable for severity would not meet the assumptions of NID(0, sigma^2) for the residuals.  In GLIMMIX, you can work around this, depending on the endpoint definition.

Steve Denham

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Posts: 25

Re: Repeated measurements analysis....Which is the best SAS procedure?

Thanks Steve for your suggestions and answer to my question. I agree with the argument since the subject were destroyed and not replaced during the analysis.

True, choosing the right model was difficult, because i choose Proc Mixed. But with the additional information, and the preference for GLIMMIX, then, i will work around and find how to carry out the analysis. Could i have more questions, i will post them out again to the SAS community.

I thank you so much for your continued support in my quest to learn more Statistical procedure. Thumbs up.

Peter

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