Accounting for Preferential Sampling Using a Constructed Covariate
In geostatistics, it is commonly assumed that sampling locations are selected independently of the underlying spatial process. In practice, however, this assumption is frequently violated. In fisheries, for example, sampling sites are often chosen to maximize expected catches, creating a stochastic dependence between the abundance process and the sampling design. Such preferential sampling can introduce substantial bias and compromise statistical inference. This study investigates the use of constructed covariates, based on average distances from nearest neighbours observations, that are able to mitigate preferential sampling. The inclusion of such covariate in the geostatistical model might be able to account for the stochastic dependence of sampling locations on the spatial variable. If this inclusion sufficiently captures the dependence, conventional methods of inference may be applied without resorting to more complex models. The proposed methodology is evaluated through an extensive simulation study that explores a variety of sampling scenarios and spatial configurations. Additionally, we demonstrate the practical utility of the approach using two real-world datasets: one on fishery landings provided by the Instituto Portugu\^es do Mar e da Atmosfera, and another concerning lead pollution biomonitoring in Galicia. Results show that incorporating the constructed covariate can substantially reduce the impact of preferential sampling, enabling reliable inference with standard geostatistical tools. We also discuss practical challenges, limitations, and paths for future methodological development.