Threats to Internal Validity
- Selection--Differences across groups may be due to pre-existing
differences in the participants.
- Maturation--In the short- and long-term, people may change,
so their responses can change from the beginning of the study to the end in
ways that are unrelated to the independent variable.
- Mortality (Attrition)--Over the course of a study, some
participants may drop out, so the final group may show differences because
of changes in the sample rather than because of the independent variable.
- History--An event that occurs outside the context of the
research may affect one group, causing a difference that seems to be related
to the independent variable but actually is not.
- Instrumentation--If the researcher changes the way he or
she measures the dependent variable, it may cause a difference in the measurement
of the dependent variable.
- Testing--When people undergo repeated testing, they may
change because of the process of testing rather than because of an independent
- Statistical Regression--Scores on an initial testing may
be extreme because of random measurement error. Subsequently, that error diminishes
or disappears, leading the the appearance that scores changed because of the
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