[R] multilevel models and sample size
Prof Brian Ripley
ripley at stats.ox.ac.uk
Sun Nov 27 19:38:33 CET 2005
On Sun, 27 Nov 2005, Berton Gunter wrote:
> "All models are wrong, but some are useful." --George Box
>
> I do not understand what you mean by "acceptable, nor "levels" nor "units".
> Specifying your model would help clarify things, I think. If by "levels" you
> mean number of different values of a random factor, than 2 levels is
> unlikely to tell you much useful about the variability of that factor. On
> the other hand, 50 values might be. Depends on the model,the data, and the
> scientific objectives, none of which you have stated clearly enough for me
> to understand, anyway.
My guess is that he means this is a tested design with e.g. 52 classes
containing 19-23 pupils each. (It always helps to state the real
problem!)
If so, this is quite a large problem for multilevel models. The classical
nested designs for measurement errors typically have two replications at
the lowest level - you get an idea of the variability from the many
differences between matched pairs. Of course the homogeneity assumptions
have to be approximately true.
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of ronggui
> Sent: Sunday, November 27, 2005 9:34 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] multilevel models and sample size
>
> It is not a pure R question,but I hope some one can give me advices.
>
> I want to use analysis my data with the multilevel model.The data has 2
> levels---- the second level has 52 units and each second level unit has
> 19-23 units.I think the sample size is quite small,but just now I can't make
> the sample size much bigger.So I want to ask if I use the multilevel model
> to analysis the data set,will it be acceptable? or unacceptable because of
> the small sample size?
--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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