Search for: All records

In many observational longitudinal studies, the outcome of interest presents a skewed distribution, is subject to censoring due to detection limit or other reasons, and is observed at irregular times that may follow a outcome-dependent pattern. In this work, we consider quantile regression modeling of such longitudinal data, because quantile regression is generally robust in handling skewed and censored outcomes and is flexible to accommodate dynamic covariate-outcome relationships. Specifically, we study a longitudinal quantile regression model that specifies covariate effects on the marginal quantiles of the longitudinal outcome. Such a model is easy to interpret and can accommodate dynamic outcome profile changes over time. We propose estimation and inference procedures that can appropriately account for censoring and irregular outcome-dependent follow-up. Our proposals can be readily implemented based on existing software for quantile regression. We establish the asymptotic properties of the proposed estimator, including uniform consistency and weak convergence. Extensive simulations suggest good finite-sample performance of the new method. We also present an analysis of data from a long-term study of a population exposed to polybrominated biphenyls (PBB), which uncovers an inhomogeneous PBB elimination pattern that would not be detected by traditional longitudinal data analysis.

In this study, a facile method is presented to fabricate superamphiphobic surfaces with controllable adhesion on polytetrafluoroethylene (PTFE), for the first time, using femtosecond laser Bessel beam. Compared to previous structures mostly based on 1D microstructure produced by Gaussian beam, the surfaces are characterized by highly uniform 2D periodic hill‐groove structures covered with extensive porous‐mesh nanostructures. Most significantly, the 2D hill‐groove structures have a very high‐aspect‐ratio since the energy distribution of the Bessel beam is more uniform over a longer focusing range. Moreover, the profile of the obtained microstructures is a nearly perfect semi‐spherical shape. As a result, the processed surfaces become superamphiphobic, exhibiting a contact angle of 166° for water and 160° for oil, respectively. Furthermore, the surface adhesion can be controlled from ultralow to ultrahigh by adjusting the period of the hill‐groove 2D‐patterned structures. It is demonstrated that the ultralow adhesion surfaces show excellent antifog and anti‐icing properties, while the ultrahigh adhesion surfaces can be used for water and oil collection. Both surfaces have a good mechanical stability and are stable over a wide range of temperatures. The superamphiphobic PTFE surfaces with tunable adhesion can be used for self‐cleaning, microfluidic systems, and harsh environments.