Sometimes in psychology experiments, groups of participants are required to
take the same test more than once. In some cases, performance can change from the first instance of testing and the next simply due to repeating the activity. In psychology research,
any change that results merely from the repetition of a task is known as
a practice effect.
https://www.explorepsychology.com/practice-effects/Practice effects occur when a participant in an experiment is able to perform a task and then perform it again at some later time. Generally, they either have a positive (subjects become better at performing the task) or negative (subjects become worse at performing the task) effect. Repeated measures designs are almost always affected by practice effects;
https://psychology.wikia.org/wiki/Practice_effects