I've been following the spacing effect for over a decade, writing a research-to-practice report in 2006, and recommending the spacing effects to my clients and in the guise of subscription learning (threaded microlearning).
One of the fascinating things is that researchers continue to be fascinated with the spacing effect producing about 10 new studies every year and many research reviews.
Here are a list of the research reviews from most recent to earliest.
Maddox, G. B. (2016). Understanding the underlying mechanism of the spacing effect in verbal learning: A case for encoding variability and study-phase retrieval.
Tags: Research Briefs