Clark's fantastic book should be of great use to trainers and instructional designers. Instead of offering an idiosyncratic series of suggestions based upon her personal experience as a trainer, Clark offers practitioners a guide to contemporary academic research in instructional strategy and the psychology of learning. The organization of this book is particularly helpful because it focuses on questions that any trainer will have encountered in their daily work: What are the most effective graphics? Where should we place the knowledge checks? Are some forms of media more effective than others? Clark helps us answer these questions by synthesizing evidence from a wide range of academic studies and experiments.
For me, her discussion of cognitive load, which determines the capacities of our working memories as learners, stood as a top take-away. Most of us have a set amount of information that we can absorb via our working memories when we learn; if more information than we can process is offered, we just don't retain it. Or, if the instructional strategy is distracting (as can be the case for lessons that prioritize behavioral engagement over psychological engagement), we also don't learn as well. Clark then analyzes the value of various instructional strategies through this useful conceptual lens.
This book should help instructional designers make better design decisions in our day-to-day jobs. To this end, Clark includes a practical appendix with a quick guide to research conclusions on various topics (communication modes, using examples, principles of engagement, etc.), which will be helpful as an occasional refresher.