Document details for 'A bootstrap estimation scheme for chemical compositional data with nondetects'

Authors Palarea Albaladejo, J., Martin-Fernandez, J.A. and Olea, R.A.
Publication details Journal of Chemometrics 28(77), 585-599.
Keywords compositional data, bootstrap, detection limits, imputation methods, logratio transformations
Abstract The bootstrap method is commonly used to estimate the distribution of estimators and their associated uncertainty when explicit analytic expressions are not available or are difficult to obtain. It has been widely applied in environmental and geochemical studies, where the data generated often represent parts of whole, typically chemical concentrations. This kind of constrained data is generically called compositional data, and they require specialised statistical methods to properly account for their particular covariance structure. On the other hand, it is not unusual in practice that those data contain labels denoting nondetects, that is, concentrations falling below detection limits. Nondetects impede the implementation of the bootstrap and represent an additional source of uncertainty that must be taking into account. In this work, a bootstrap scheme is devised that handles nondetects by adding an imputation step within the resampling process, and conveniently propagates their associated uncertainly. In doing so, it considers the constrained relationships between chemical concentrations originated from their compositional nature. Bootstrap estimates using a range of imputation methods, including new stochastic proposals, are compared across scenarios of increasing difficulty. They are formulated to meet compositional principles following the log-ratio approach, and an adjustment is introduced in the multivariate case to deal with non-closed samples. Results suggest that nondetect bootstrap based on model-based imputation is generally preferable. A robust approach based on isometric logratio transformations appears to be particularly suited in this context. Computer routines in the R statistical programming language are provided.
Last updated 2014-07-31

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