Have you ever discovered an error in the DI programs? The programs are thoroughly vetted and field tested before publication, but sometimes errors still appear.
If you've found an error in a DI program, email the authors at firstname.lastname@example.org. The Engelmann-Becker Corporation (named after the founders of DI) is devoted to designing, field testing and refining the DI curricula both before and after they are published. The E-B Corp would be eager to learn of any errors you've found in the programs so they can note them and work with the publisher to correct errors before the next printing.