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From | "Airey, David C" <david.airey@vanderbilt.edu> |
To | "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |
Subject | st: glm for binomial regression with |
Date | Wed, 20 Apr 2011 13:46:08 -0500 |
. I see the cloglog link in xtgee, and I have just one level of clustering, so this is a possibility. > I have questions about binomial regression. > > On page 527 of the Stata 11 -glm- help in the [R] base reference PDF manual is described in Example 2 a binomial data set which describes the death of beetles for a dose response experiment (ldose = log dose, n = total number of beetles, r = number dead): > > . list , clean > > ldose n r > 1. 1.6907 59 6 > 2. 1.7242 60 13 > 3. 1.7552 62 18 > 4. 1.7842 56 28 > 5. 1.8113 63 52 > 6. 1.8369 59 53 > 7. 1.861 62 61 > 8. 1.8839 60 60 > > The data is modeled by: > > glm r ldose, family(binomial n) link(logit) > > or > > glm r ldose, family(binomial n) link(cloglog) > > where the cloglog links allows the dose curve to be asymmetric. In these data the cloglog link fits better than the logit link. > > I have data like the above, except with replications at each dose. > > The manual also says the data could be analyzed by expanding the data and using -logit- (if the logit link was the better fit). > > I have two questions. > > Unlike the data above, I have replications for each dose. Is this -xt- or clustered data? > > The data above are already grouped and beetles are replicates, but we have: > > . list , clean > > ldose n r > 1. 1.6907 59 6 > 2. 1.6907 62 5 > 3. 1.6907 62 10 > 4. 1.6907 59 3 > etc. > > I could ignore the potential clustering and simply model n = 59+62+62+59 and r = 6+5+10+3. I guess it depends on how the experiment is actually done, and I could test for clustering too. > > My second question, however, is if I were to expand the data above that included a replication by dose (with appropriate replicate id variable included as the cluster id), I could analyze this using xtlogit or xtmelogit---but how do you do this if you want asymmetry, like you get with glm and the cloglog link? Is that available in glm but not xt models in Stata? > > -Dave * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/