(1) Dipartimento di Scienze Statistiche Paolo Fortunati, Università di Bologna, Via delle Belle Arti, 41, I-40127 Bologna, Italy;(2) Dipartimento di matematica, statistica, informatica e applicazioni, Università di bergamo, Italy
Abstract:
We consider the problem of assessing long-term trends of ozone concentrations measured on a single site located in an urban area. Among the many methods proposed in the literature to eliminate the confounding effect of changing weather conditions, we employ a stratification of daily maxima based on regression trees. Within each stratum conditional independence and Weilbull distribution are assumed for maxima. Long-term trend is defined non-parametrically by the sequence of yearly medians. Models are estimated following the Bayesian approach. The alternative assumptions of common and stratum specific trends are compared and a model with common trend for all strata is selected for the analyzed real dataset. The conditional independence assumption is checked by the comparison with a model including an autoregressive component.