Associate Professor of Biostatistics
Ph.D., 2001, Free University of Amsterdam, The Netherlands
Dr. Judith Lok’s research program currently focuses on statistical methods in HIV-AIDS related problems, using observational data. Together with Dr. James Robins and Dr. Miguel Hernan she is working on estimating the optimal start of HAART treatment in HIV positive patients. The main issue there is that patients who are treated and patients who are not treated are not comparable. Together with Dr. Ron Bosch and Dr. Michael Hughes she has been working on the course of CD4 after treatment in the long run (7-8 years), using the ALLRT data. The main issue there is that not all patients who started treatment also entered into ALLRT, leading to considerable dropout. Together with Dr. Ron Bosch, Dr. Mireille Schnitzer, and clinicians she is working on how immune activation after HAART treatment predicts subsequent AIDS-defining and non-AIDS-defining events as well as immune function, using the ALLRT data. To accomplish this, they have developed efficient and doubly robust estimation of a prediction model for events in the presence of informative censoring. Together with Dr. Victor DeGruttola and Dr. James Robins she is working on estimating the effect of HAART as a function of time since infection. Also there, the main issue is that patients who are treated and patients who are not treated are not comparable. To accomplish this, they have developed optimal estimation of coarse Structural Nested Mean Models. Together with Dr. Michael Hughes and Brian Sharkey she is working on safety outcomes in HIV clinical trials, using a competing risks model estimated in the presence of informative censoring. Together with Constantin Yiannoustos, Agnes Kiragga, and Ron Bosch she is estmating the CD4 count trajectory after HAART initiation in Kenya. For this, she has developed Inverse Probability of Censoring Weighting when the data are Missing Not At Random, but when an “outreach” sample is available: data on a selection of the patients who dropped out. In addition, she is working on mediation. She has proposed a new definition of direct and indirect effects, “organic” direct and indirect effects, an intervention based approach which obviates the need to be able to “set” the mediator to a specific value. Its assumptions are implied by the usual assumptions to identify natural direct and indirect effects.
Judith Lok developed an intellectual and mathematical framework for Structural Nested Models, both for continuous time measurements and for discrete time measurements. In collaboration with Dr. Richard Gill, Dr. Aad van der Vaart and Dr. James Robins, she provided a rigorous proof of the asymptotic properties, consistency and asymptotic normality, of the resulting estimators in discrete time, which was published in Statistica Neerlandica. She has also provided the first proofs of these asymptotic properties in continuous time. This work has been published in the Scandinavian Journal of Statistics, “Structural nested models and standard software: a mathematical foundation through partial likelihood” and in the Annals of Statisics, “Statistical modelling of causal effects in continuous time”. These methods have been applied to estimate treatment effects in HIV infected subjects. Judith Lok is still working on structural nested models in continuous time.
Research areas: causality, counterfactuals, longitudinal data, observational studies, competing risks, survival analysis, HIV/AIDS.
cvam20 – Judith Lok’s Curriculum Vitae
Biosketch– Judith Lok’s Biosketch