Dr. Chiu, a reading specialist, wants to know whether third graders who learned to read predominantly by phonics are better at comprehending text than children who learned to read predominantly by whole language. To find out, she decides to randomly assign two groups of first graders to either a phonics-based reading program or a whole language-based reading program. She will then give each group a reading comprehension test when they reach third grade. This study provides an example of a(n):