title: An index of local sensitivity to nonignorable drop-out in longitudinal modelling.
abstract: In longitudinal studies with potentially nonignorable drop-out, one can assess the likely effect of the nonignorability in a sensitivity analysis. Troxel et al. proposed a general index of sensitivity to nonignorability, or ISNI, to measure sensitivity of key inferences in a neighbourhood of the ignorable, missing at random (MAR) model. They derived detailed formulas for ISNI in the special case of the generalized linear model with a potentially missing univariate outcome. In this paper, we extend the method to longitudinal modelling. We use a multivariate normal model for the outcomes and a regression model for the drop-out process, allowing missingness probabilities to depend on an unobserved response. The computation is straightforward, and merely involves estimating a mixed-effects model and a selection model for the drop-out, together with some simple arithmetic calculations. We illustrate the method with three examples.
authors.affiliation: Clinical Biostatistics, Merck & Co. Inc., BL3-2, Blue Bell, PA 19422, USA. guoguang_ma@merck.com
authors.forename: Guoguang
authors.initials: G
authors.lastname: Ma
authors.forename: Andrea B
authors.initials: AB
authors.lastname: Troxel
authors.forename: Daniel F
authors.initials: DF
authors.lastname: Heitjan
mesh_terms: D000821:Animal Feed; D000818:Animals; D000929:Antidepressive Agents, Tricyclic / Q000627:therapeutic use; D018931:Antineoplastic Agents, Hormonal / Q000627:therapeutic use; D002417:Cattle; D019970:Cocaine-Related Disorders / Q000150:complications / Q000191:economics; D003627:Data Interpretation, Statistical; D003866:Depressive Disorder / Q000150:complications / Q000188:drug therapy; D003891:Desipramine / Q000627:therapeutic use; D005260:Female; D005485:Flutamide / Q000627:therapeutic use; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008894:Milk Proteins / Q000378:metabolism; D015233:Models, Statistical; D010352:Patient Dropouts; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000523:psychology; D011788:Quality of Life; D016032:Randomized Controlled Trials as Topic / Q000379:methods*
chemical_list: D000929:Antidepressive Agents, Tricyclic; D018931:Antineoplastic Agents, Hormonal; D008894:Milk Proteins; D005485:Flutamide; D003891:Desipramine