Upon fitting the mixed model 3 to the example data and calculating the semi-Bayes estimates from model 4, estimates that were unstable and nonsensical became more precise and reasonable, whereas those that were relatively precise did not change much. For example, the maximum-likelihood estimate of the OR for eating 100 additional grams of white cabbage was 2.5 (95% CI = 0.75–8.4); in contrast, the semi-Bayes OR estimate from GLIMMIX was 1.2 (95% CI = 0.59–2.6) (Table 1). In contrast, the OR for eating 100 additional grams of allium vegetables changed little upon applying the multilevel model (Table 1). Setting the elements of τ to larger values (for example, 0.53, which corresponds to a 95% prior certainty that the residual relative risk lies in an eightfold range) gave less stable results; here, the semi-Bayes OR for eating 100 additional grams of white cabbage was 1.7 (95% CI = 0.64–4.4). Using the GLIMMIX default (that is, empirical-Bayes) led to the common τ2 being estimated as 0, which ignores any residual second-stage effects and results in overly precise estimates that are pulled too close together. 5
The Appendix provides the SAS IML code used to calculate ORs and 95% CIs from our GLIMMIX output. The complete program, a corresponding SAS macro, and instructions on how to obtain results using a multilevel model with GLIMMIX, are available at URL http://darwin.cwru.edu/∼witte/hm.html.
With the explication and code given here, epidemiologists can use GLIMMIX to analyze their own data with a multilevel model. Strengths of GLIMMIX include allowing for more than two stages and providing diagnostic statistics. A recent report 26 provides further evaluation of GLIMMIX and comparison with variance component software packages specifically written for multilevel modeling. 27–29 As with SAS GLIMMIX, however, use of these packages for epidemiologic analysis will require either writing special code or altering output to give the relative risk estimates and corresponding confidence intervals. 20 Multilevel modeling can also be undertaken with procedures available in the SAS IML and GAUSS languages. 5,6,20,30 These procedures provide standard epidemiologic output and are available from URL http://darwin.cwru.edu/∼witte/hm.html.
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