Classroom teachers need to make sound judgments and decisions concerning curricular and instructional issues. The teachers who wish to become more effective in the classroom should learn to develop their own research designs since educational research reported in journals is often inconclusive, conflicting, or not relevant to the classroom teachers. Experimental studies can be particularly helpful to teachers if they investigate the effects of variables such as textbooks, tests, and homework assignments on one or more other variables such as student knowledge, student attitudes, and length of time required to complete a test or homework assignment. Factors to be considered when designing an experimental study include experimental validity (comparing and contrasting results with results from a control group or situation), internal validity (extent to which the observed effect appears to be due to one's experimentation), and external validity (determination of persons and circumstances to which the results apply). The conclusion is that teachers can do valid classroom research if they combine simple descriptive statistics with common sense and exercise care in maintaining experimental validity when they gather information. (DB)