Answer:
Maturation effect
Explanation:
Maturation is a factor that threatens the internal validity. The experiment does not happen overnight. The experiment can be happened in a couple of days, weeks, year and so on.
The dissertation experiment does not run for a long time. Many of the maturation effects occur within a very short period. It can occur within a few days. Many factors occur with subjects such as tiredness, boredom, and hunger inattention, etc.
Thus Emma finds that there was a significant increase in their vocabulary. She does not conclude that it happened because of the maturation effect.