This Report is an expansion of a previous Report on the DARPA XAI Program, which was titled
"Literature Review and Integration of Key Ideas for Explainable AI," and was dated February
2018. This new version integrates nearly 200 additional references that have been discovered.
This Report includes a new section titled "Review of Human Evaluation of XAI Systems." This
section focuses on reports—many of them recent—on projects in which human-machine AI or
XAI systems underwent some sort of empirical evaluation. This new section is particularly
relevant to the empirical and experimental activities in the DARPA XAI Program
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