Estimation of policy effects using parametric nonlinear models: a contextual critique of the generalized method of moments

The typical empirical study in health services and outcomes research is aimed at estimating the causal effect that an exogenously imposed condition (e.g. a policy mandate) will have (or has had) on a specified outcome of interest. Controlling for unobservable confounding influences is of primary importance in such analyses. The instrumental variables (IV) method has been widely used for this purpose in the linear regression context. The present paper examines the pros and cons of alternative versions of the generalized method of moments (GMM) [of which the IV estimator is a special case] for the estimation of policy effects when endogeneity is present in a nonlinear regression setting. We show that conventional GMM is difficult to implement for policy analysis because it does not typically accommodate symmetry—similar treatment of both observable and unobservable confounders in the regression specification. Although, simple additive (nonsymmetric) regression specifications afford practical GMM estimators, they are difficult to defend from both intuitive and conceptual standpoints. Moreover, as we show via simulation, if symmetry is ignored and conventional GMM is applied based on an incorrectly specified non-symmetric model, then policy analytic estimates can be seriously biased. As a result, prospects for the development and application of intuitive consistent GMM-based policy effect estimators are dim. The problem stems from the reasonable desire on the part of the researcher to derive GMM estimators in the nonlinear framework that are based solely upon the conventional minimalist linear IV assumptions. We show, in the context of our formulation of a simple but consistent alternative to GMM in the probit case, that intuitively appealing additional assumptions about the data generating process of the policy variable will often be sufficient for the development of desirable alternatives to the GMM.     

Renal Allograft Biopsies with Borderline Changes: Predictive Factors of Clinical Outcome

The clinical outcome and appropriate management for patients showing 'borderline changes' on allograft biopsy after renal transplantation is still controversial. In an attempt to identify predictive factors of clinical outcome of patients with such lesions, we reviewed the clinical course of 91 patients with borderline changes. Multivariate analysis revealed significant and independent effects of histological stage (i + t < or = or > 2) and time to borderline changes (< or = or > 3 months after transplant) on serum creatinine levels at 1 year from borderline changes episodes (respectively, p = 0.04 and p = 0.02) and only a significant effect of time to borderline changes on serum creatinine levels at 2 years (p = 0.005). Renal function at 1 year and 2 years as 5- and 8-year graft survival were not significantly different in the group of patients treated with antirejection therapy (T group, n = 49) compared with the untreated group (UT group, n = 42). This study strongly suggests that borderline changes with histological score (i + t) > 2 and late episodes of borderline changes should be considered to be of poor prognosis.     

Internal thoracic vessels as recipient vessels for free flap reconstruction in head and neck surgery.

In rare cases the usage of the internal thoracic vessels as recipient vessels in reconstructive surgery of the head and neck region with free tissue transfer is a challenging but valid alternative if local recipient vessels are unusable.     

Successful closure of spontaneous esophageal perforation (Boerhaave's syndrome) by endoscopic clipping.

Endoscopic clips have been used mainly for control of gastrointestinal (GI) bleeding and occasionally for closure of GI perforations. However, closure of spontaneous esophageal perforation (Boerhaave's syndrome) by clipping has not been reported. We described successful non-surgical closure of spontaneous esophageal perforation by endoscopic clipping in a patient with bilateral pyopneumothorax and septicemia.