Imputes missing biomarker measurements in a wide longitudinal
serum panel with gradient-boosted decision trees, groups the completed
panel by Bayesian consensus clustering, and compares the resulting patient
subgroups by Kaplan-Meier, log-rank and Cox analysis. The imputation
learner is described in Ke et al. (2017)
< https://papers.nips.cc/paper/6907-lightgbm-a-highly-efficient-gradient-boosting-decision-tree>
and the clustering method in Lock and Dunson (2013)