Although the "Don't Know" (DK) option has received telling criticism in maximum performance summative tests, its potential use in formative evaluation was considered and judged to be more promising. The pretest of an instructional module was administered with DK options. Examinees were then required to answer each question to which they had responded DK. Concurrent validity of the resulting measures indicated superiority (p. less than .05) of the procedure using the DK option for one of the two criterion measures investigated. Results also revealed a need to teach students to use the option when it is used in formative evaluation. (Author)