Internal validity is important in assessing teaching demonstrations both for one's knowledge and for quality assessment demanded by outside sources. We describe a method to improve the internal validity of assessments of teaching demonstrations: a 1-group pretest-posttest design with alternative forms. This design is often more practical and ethical than random assignment, and it is more valid than a single posttest-only or simple 1-group pretest-posttest design. We describe how to interpret results from this design and discuss the advantages and disadvantages. (Contains 1 figure.)