Benchmark 9E
The Mathematical World: Reasoning
Grades 6-8, page 233

A single example can never prove that something is always true, but sometimes a single example can prove that something is not always true.
 

NSES Content Standard A 
Science as Inquiry: Abilities necessary to do scientific inquiry
Grades 5-8, page 145
Think critically and logically to make the relationships between evidence and explanations. Thinking critically about evidence includes deciding what evidence should be used and accounting for anomalous data. Specifically, students should be able to review data from a simple experiment, summarize the data, and form a logical argument about the cause-effect relationships in the experiment. Students should begin to state some explanations in terms of the relationship between two or more variables.