@article{kendall_williams_hermansen_bois_thanh_2024, title={Beyond Time-Homogeneity for Continuous-Time Multistate Markov Models}, ISSN={["1537-2715"]}, DOI={10.1080/10618600.2024.2388609}, abstractNote={Multistate Markov models are a canonical parametric approach for data modeling of observed or latent stochastic processes supported on a finite state space. Continuous-time Markov processes describe data that are observed irregularly over time, as is often the case in longitudinal medical data, for example. Assuming that a continuous-time Markov process is time-homogeneous, a closed-form likelihood function can be derived from the Kolmogorov forward equations – a system of differential equations with a well-known matrix-exponential solution. Unfortunately, however, the forward equations do not admit an analytical solution for continuous-time, time-inhomogeneous Markov processes, and so researchers and practitioners often make the simplifying assumption that the process is piecewise time-homogeneous. In this paper, we provide intuitions and illustrations of the potential biases for parameter estimation that may ensue in the more realistic scenario that the piecewise-homogeneous assumption is violated, and we advocate for a solution for likelihood computation in a truly time-inhomogeneous fashion. Particular focus is afforded to the context of multistate Markov models that allow for state label misclassifications, which applies more broadly to hidden Markov models (HMMs), and Bayesian computations bypass the necessity for computationally demanding numerical gradient approximations for obtaining maximum likelihood estimates (MLEs). Supplemental materials are available online.}, journal={JOURNAL OF COMPUTATIONAL AND GRAPHICAL STATISTICS}, author={Kendall, Emmett B. and Williams, Jonathan P. and Hermansen, Gudmund H. and Bois, Frederic and Thanh, Vo Hong}, year={2024}, month={Sep} }
 @article{volpe_kendall_collins_graham_williams_holochwost_2024, title={Prior exposure to racial discrimination and patterns of acute parasympathetic nervous system responses to a race-related stress task among Black adults}, volume={10}, ISSN={["1469-8986"]}, DOI={10.1111/psyp.14713}, abstractNote={Abstract Black adults' prior exposure to racial discrimination may be associated with their acute parasympathetic reactivity to and recovery from a new race‐related stressor. Existing analytical approaches to investigating this link obscure nuances in the timing, magnitude, and patterns of these acute parasympathetic nervous system (PNS) responses. In a re‐analysis of a prior study, we utilize an hidden Markov model (HMM) approach to examine how prior experiences of racial discrimination are associated with intraindividual patterns of (1) physiological states of PNS activity and (2) patterns of and variability in transitions between these physiological states. Participants ( N = 118) were Black young adults (range 18–29 years; M age = 19.67, SD age = 2.04) who completed an online survey to index prior racial discrimination exposure, followed by an in‐person lab visit during which their PNS activity in response to a race‐related stress task was measured via electrocardiogram and converted into respiratory sinus arrhythmia. HMMs indicated evidence for two states: baseline and a second state representing a significant reduction in respiratory sinus arrhythmia. Most participants (93.22%) demonstrated a blunted response to the task, indicating that they did not transition from baseline during the procedure. Prior racial discrimination was not associated with HMM states or state transition parameters. Blunted physiological responses may be an important area of future investigation that could inform early life course mental and physical health screenings.}, journal={PSYCHOPHYSIOLOGY}, author={Volpe, Vanessa V. and Kendall, Emmett B. and Collins, Abbey N. and Graham, Matthew G. and Williams, Jonathan P. and Holochwost, Steven J.}, year={2024}, month={Oct} }