Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2013/iii/paper-30/5/b/iv/solution

The preferred cumulative-response logistic model estimates
Its logit link describes conditional smoking-free probability given baseline predictors and prior successful weeks. Holding the other predictors fixed, males have times the female success odds; the reported p-value is , and the approximate 95% odds ratio confidence interval is . Age has estimated odds ratio per additional year, with p-value , giving little evidence for an age association in this fit.
The combined treatment has conditional success odds ratio versus the reference treatment, with approximate 95% confidence interval and p-value . The fitted conditional odds are about 39% higher for the combined treatment. The trial randomization supports treatment comparisons, but conditioning on accumulated post-treatment outcomes means this coefficient is not directly the marginal total treatment effect.
Each previous successful week multiplies current success odds by , with approximate 95% confidence interval and very small p-value . This is strong fitted persistence. It can reflect true state dependence, unobserved heterogeneity, or an omitted calendar-time trend; this fit alone cannot distinguish them. The baseline intercept implies success probability for a reference-treatment female aged zero with no previous success. That age is outside the study's useful interpretation range, so the intercept chiefly anchors the regression. The Bernoulli distribution dispersion parameter is fixed at one, and the residual binomial deviance is 1122 on 995 statistical degrees of freedom. With individual binary outcomes, comparing that binomial deviance mechanically to a chi-squared distribution is not a reliable general goodness-of-fit test.

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