Benchmark 12E
Habits of Mind: Critical-Response Skills
Grades 3-5, page 299

Recognize when comparisons might not be fair because some conditions are not kept the same.

NSES Content Standard A 
Science as Inquiry: Abilities necessary to do scientific inquiry
Grades K-4, page 122
Plan and conduct a simple investigation. In the earliest years, investigations are largely based on systematic observations. As students develop, they may design and conduct simple experiments to answer questions. The idea of a fair test is possible for many students to consider by fourth grade.