Final answer:
When subtest scaled scores for an Index differ by more than 5 points, further investigation is needed to understand the reasons behind the discrepancy, which could include uneven skills or testing errors.
Step-by-step explanation:
When the difference between the lowest and highest subtest scaled scores for an Index is greater than 5 points, the correct answer is c) Requires further investigation. This variance in scores could indicate uneven skill development, testing errors, or other factors that may have impacted the test results. It does not necessarily mean that the testing is invalid; instead, it signals a need for deeper analysis to understand the reasons behind such a discrepancy. A thorough investigation might reveal specific areas where a student needs support or whether an external factor influenced their performance on the subtests.