Question: Why do we need to ensure we have multiple parallel or equated forms when measuring student progress? 

Answer: The crucial issue with multiple forms is that classic problem of educational measurement. In intensive intervention we want to measure children repeatedly over a fairly short duration—say every two weeks, every one week—and so we want equal forms so what we see in terms of children’s progress is because of their actual progress or the results of our intervention rather than just the form.  So if we give an easy form, we would imagine a high rate of growth. If we give hard forms we would imagine maybe even a negative form of growth. That is undesirable. We want forms to be equal.  That being said, forms are never perfectly equal and so we need to have forms that are designed to be as close as possible, as equivalent as possible, and then we also need estimation methods—models, statistical models—to equate them. And so equating becomes important also. So that when we measure children’s progress—when we measure their growth—we can be sure that it is true growth and not just an accident of the form. And that’s why alternate forms are important, because we need their repeated use and we need them to be as close as possible by design as well as empirically from our methods.