Past exam of the mathematics course of the University of Cambridge / 2012 / iii / Paper 37 / 6 / b / Solution

For the moment approach, write $m_i=\mathbb{E}{Y}_i$ and let $V_i$ be a positive-definite working covariance constructed from the proposed moment formulas. For identifiable mean coefficients $\theta$, let $D_i=\partial m_i/\partial {\theta }^T$. Generalized estimating equations take the formAcross independent students, correct mean specification and standard regularity conditions give consistent identifiable mean coefficients even if the working covariance is wrong. Sandwich covariance matrix standard errors useResidual second-moment estimating equations can estimate identifiable dispersion and association parameters. The marginal mean only identifies ${\gamma }_0={\beta }_0+\log (1-\pi )$, not ${\beta }_0$ and $\pi$ separately. Moment restrictions beyond the mean would be needed to separate them; as shown below, the printed restrictions do not generally recover the intended latent parameters.

For the hierarchical approach, maximize the observed-data likelihood after integrating out the random effect. Set ${\mu }_{ij}=e^{{\beta }_0+{\beta }^T{x}_i+{\alpha }_j}$, $r=1/\tau$, $M_i={\sum }_j{\mu }_{ij}$ and $s_i={\sum }_j{y}_{ij}$. The joint likelihood contribution isThe integral comes from multiplying three conditional Poisson mass functions by the Gamma density and integrating $b_i^{r+s_i-1}{e}^{-(r+M_i)b_i}$ over $b_i>0$. Independent students give $\ell ={\sum }_i\log L_i$. Direct numerical maximum likelihood or an expectation-maximization algorithm can be used; in an EM algorithm an all-zero profile has an uncertain latent component membership. Information-based likelihood standard errors apply under regular interior conditions, with special care for parameter boundaries.