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maturation
an experimental group improves over time only because of natural development or spontaneous improvement.
history
an experimental group changes over time because of an external factor that affects all or most members of the group
regression to the mean
an experimental group whose average is extremely low or high at pretest will get better or worse over time because the random events that caused the extreme pretest scores do not recur the same way at post test
attrition
an experimental group changes over time, but only because the most extreme cases have systematically dropped out and their scores are not included in posttest
testing
a type of order effect: an experimental group changes over time because repeated testing has affected the participants.
instrumentation threat
an experimental group changes over time, but only because the measurement instrumentation has changed.
observer bias
an experimental group’s ratings differ from a comparison group, but only because the researcher expects the groups’ ratings to differ
demand characteristic
participants guess what the study’s purpose is and change their behavior in the expected direction.
Note 
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