threats to internal validity

Maturation

• A change in behavior that emerges more or less spontaneaously over time

History

• Result from a  historical or external factor that systematically affects most members of the treatment group at the same time as the treatment itself

Regression

• Regression to the mean

• Group average is unusually extreme and time 1 but time 2 is less extreme

Attrition

• Problem when attrition is systematic (only a certain kind of participant drops out)

• Happens when pretest and posttest are administered on separate days and some participants are not available on the second day

Testing

• A change in the participants as a result of taking a test more than once

• Causes people to become more practiced at taking the test

Instrumentation

• Occurs when a measuring instrument changes over time

Observer bias 

• When researchers’ expectations influence their interpretation of the results

Demand Characteristics 

• A problem when participants guess what the study is supposed to be about and change their behavior in the expected direction

 When a study results in null effects

• A finding that an independent variable did not make a difference in the dependent variable