Benchmark 1A
The Nature of Science: The Scientific World View
Grades 3-5, page 6

Results of similar scientific investigations seldom turn out exactly the same. Sometimes this is because of unexpected differences in the things being investigated, sometimes because of unrealized differences in the methods used or in the circumstances in which the investigation is carried out, and sometimes just because of uncertainties in observations. It is not always easy to tell which.

See the general discussion in Unifying Concepts and Processes: Systems, order, and organization.