Forty 9-year-olds were studied for 3 years and their attention to tasks and reading comprehension were assessed every month. The researchers implemented an academic program and detail-to-task program for each child over the course of the 3 years. The researchers published a paper noting that there were tremendous increases in attention to detail and reading comprehension from when the child was 9 years old to when the child was 12 years old. Researchers need to be careful claiming credit for this change because _____ may also account for the change.