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IJSMRV1N1A02

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IJSMRV1N1A02

Enriched-Data Problems and Essential Non-Identifiability - Pages 16-44
Geert Molenberghs1,2, Edmund Njeru Njagi1, Michael G. Kenward3 and Geert Verbeke1,2

1I-BioStat, Universiteit Hasselt, B-3590 Diepenbeek, Belgium; 2I-BioStat, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium; 3Medical Statistics Unit, London School of Hygiene and Tropical Medicine, London WC1E7HT, UK

http://dx.doi.org/10.6000/1929-6029.2012.01.01.02

 

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    Abstract: There are two principal ways in which statistical models extend beyond the data available. First, the data may be coarsened, that is, what is actually observed is less detailed than what is planned, owing to, for example, attrition, censoring, grouping, or a combination of these. Second, the data may be augmented, that is, the observed data are hypothetically but conveniently supplemented with structures such as random effects, latent variables, latent classes, or component membership in mixture distributions. These two settings together will be referred to as enriched data. Reasons for modelling enriched data include the incorporation of substantive information, such as the need for predictions, advantages in interpretation, and mathematical and computational convenience. The fitting of models for enriched data combine evidence arising from empirical data with non-verifiable model components, i.e., that are purely assumption driven. This has important implications for the interpretation of statistical analyses in such settings. While widely known, the exploration and discussion of these issues is somewhat scattered. The user should be fully aware of the potential dangers and pitfalls that follows from this. Therefore, we provide a unified framework for enriched data and show in general that to any given model an entire class of models can be assigned, with all of its members producing the same fit to the observed data but arbitrary regarding the unobservable parts of the enriched data. The implications of this are explored for several specific settings, namely that of latent classes, finite mixtures, factor analysis, random-effects models, and incomplete data. The results are applied to a range of relevant examples.

    Keywords: Compound-symmetry, empirical bayes, enriched data, exponential random effects, gamma random effects, linear mixed model, missing at random, missing completely at random, non-future dependence, pattern-mixture model, selection model, shared-parameter model.

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