Students make errors in mathematics for a variety of reasons. It is the job of the educator to identify the source of these errors and to assist students in correcting them. The purpose of this monograph is to help identify, analyze, and evaluate students' computational proficiency. This monograph is divided into four sections: "Whole Numbers"; "Fractions"; "Decimals"; and "Percents." Each section is arranged according to specific computational skills. Each computational skill is examined via the following components: (1) skill (giving the specific computational skill to be examined; (2) example (offering a sample problem that could lead to typical student errors); (3) entry conditions (listing the mathematical prerequisites for mastery of the skill being examined); (4) correct solution (showing correct answer to the given sample); (5) errors, identification and analysis (highlighting typical student errors and analyzing their possible source); and (6) practice (providing problems similar in scope to the example). (YP)