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 design that are able to deal with such situations. (2 of 4)

Standard 1-4 page 75, Grades 5-8
Verify and interpret results with respect to the original problem situation
 

What people expect to observe often affects what they actually do observe. Strong beliefs about what should happen in particular circumstances can prevent them from detecting other results. Scientists know about this danger and take steps to try and avoid it when designing investigations and examining data. One safeguard is to have different investigators conduct independent studies of the same question. (3 of 4)

Standard 1-4 page 75, Grades 5-8
Verify and interpret results with respect to the original problem situation