Benchmark 1B
The Nature of Science: Scientific Inquiry
Grades 6-8, page 12

If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables. It may not always be possible to prevent outside variables from influencing the outcome of an investigation (or even to identify all of the variables), but collaboration among investigators can often lead to research designs that are able to deal with such situations.
 

NSES Content Standard A 
Science as Inquiry: Abilities necessary to do scientific inquiry 
Grades 5-8, page 145 
Design and conduct a scientific investigation. Students should develop general abilities, such as systematic observation, making accurate measurements, and identifying and controlling variables. They should also develop the ability to clarify their ideas that are influencing and guiding the inquiry, and to understand how those ideas compare with current scientific knowledge. Students can learn to formulate questions, design investigations, execute investigations, interpret data, use evidence to generate explanations, propose alternative explanations, and critique explanations and procedures. 

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.