Further on migraine headaches:
(a) A graph of the fixed effects for the mixed-effects logit model fit to the migraine headaches data is shown in Figure 24.1 (page 747), and the estimated parameters of the model are given on page 746. Explain how the lines on the graph, showing how the fitted probability of headache occurrence depends on medication group and time, can be computed from the estimates of the fixed effects.
(b) As mentioned (footnote 5 on page 745), the original analysis of the migraine headaches data used a regression spline, rather than a square-root transformation, for time posttreatment. Reanalyze the data using a regression spline for time posttreatment, and compare the results to those produced by the model employed in the text.20
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