For each challenge we included a reference and/or null method set of predictions in the analysis to provide perspective for performance evaluations of blind predictions. Null models or null predictions employ a model that is not expected to be useful and can provide a simple point of comparison for more sophisticated methods, as ideally, such methods should improve on predictions from a null model. Reference methods are not formally part of the challenge, but are provided as comparison methods. For the log P challenge we included a null prediction set which predicts a constant log P value of 2.66 for every compound, as described in a previous SAMPL paper . For log D evaluation we included a set of null predictions that all of the molecules partition equally between the water and octanol phase.