The choice when expressing numbers is between using a word to express the number (e.g., three) or a figure (e.g., 3).  Please check whether or not the number is expressed correctly.  Then  highlight the rule that governs the use of the number in this instance.

After studying the material with the assigned technique, participants took immediate recall and recognition tests and then were retested one week later on the material.

The expression of the number is   correct     incorrect       because: