Incremental rehearsal is a solid intervention that can be used for letters, words, math facts, history facts, or really any types of facts.
This document provides information about how to use the data collection sheet. Example using retention and acquisition rates included.
For more information about incremental rehearsal - http://ebi.missouri.edu/wp-content/uploads/2011/03/Incremental-Rehearsal-Intervention-Brief-2.pdf