I am trying to check the significance of counselling in the technique for inhaler usage (rotahaler only) at different points of time in same subjects.

The subjects' baseline technique will be assessed using a checklist scoring : zero for incorrect/ missed step and 1 for correct step. The total score is 8 (i.e., the checklist has 8 steps). Then subject will be counselled and an immediate post intervention score is taken followed by another assessment after 2 weeks.

I want to compare the means at 3 points in time. Can I use a repeated measures ANOVA here? If not, which test is suitable?


Repeated measures ANOVA makes assumptions that may not be met; multi-level modeling makes fewer assumptions. In particular, RM-ANOVA assumes sphericity.

There was a thread on this

There is also a nice section on RM-ANOVA vs. MLM and other methods in Hedeker and Gibbons. They write "ANOVA for repeated measures assumes compound symmetry which implies constant variances and covariances over time. Clearly, such an assumption has little, if any, validity for longitudinal data".

  • $\begingroup$ That is not true. Repeated measures Anova requires specification of a covariance structure. Sphericity is only one possible choice. In SAS there are many options including AR(1), ARMA(1,1), compound symmetry, Toeplitz and even unstructured if there is enough data to allow it. $\endgroup$ – Michael R. Chernick Aug 14 '12 at 21:30
  • $\begingroup$ Wolfinger and Cheng of SAS seem to indicate otherwise. In GLM, there is no "select covariance structure" in MIXED there is. On p. 3 they say: PROC MIXED provides you with a variety of possible structures to choose from in addition to the Type H and unstructured matrices used by PROC GLM. These include compound symmetry, autoregressive andother time series structures, random coefficients models,and spatial correlations. It also points out other advantages of MIXED, such as using all the data even if some is missing. $\endgroup$ – Peter Flom - Reinstate Monica Aug 14 '12 at 21:47
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    $\begingroup$ Go argue with the SAS authors and with Hedeker and Gibbons, who I cited. I'm done. $\endgroup$ – Peter Flom - Reinstate Monica Aug 14 '12 at 22:40
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    $\begingroup$ Before I stop and possibly take this to discussion I must state that proc mixed does repeated meausres ANOVA for the mixed model. You can check this in SAS Help. $\endgroup$ – Michael R. Chernick Aug 14 '12 at 22:45
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    $\begingroup$ (+1) I don't know anything about SAS but I think the contended portion of this answer is true - see here. $\endgroup$ – Macro Aug 14 '12 at 23:12

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