General Considerations

Until the purpose of the observation becomes clear, i.e. until the student reads part 2, then any varying attribute of the strings, such as color, diameter, material, or tension, could be recorded. Students who do not record, say, color, either do not see any difference between the strings, or may have read part 2 before completing part 1. They presumably reject color on the grounds of relevance and so they are applying their scientific knowledge to the question. Part 2 is about identifying relevant variables - frequency first, string diameter second, and then describing their relationship, i.e. sound frequency varies with string diameter, the direction of the relationship necessary so that "as the diameter of the string increases, so the frequency of the sound decreases" becomes the type of answer expected.