| contamination | Occurs when there is communication of information about the experiment between groups of participants.
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| history | The occurrence of an event other than the treatment can threaten internal validity if it produces changes in the research participants’ behavior.
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| instrumentation | Changes over time can take place not only in the participants of an experiment, but also in the instruments used to measure the participants’ performance. Changes due to instrumentation can threaten internal validity if they cannot be separated from the effect of the treatment.
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| maturation | Change associated with the passage of time per se is called maturation. Changes participants undergo in an experiment that are due to maturation and not due to the treatment can threaten internal validity.
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| nonequivalent control group design | Quasi-experimental procedure in which a comparison is made between control and treatment groups that have been established on some basis other than through random assignment of participants to groups.
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| program evaluation | Research that seeks to determine whether a change proposed by an institution, government agency, or another unit of society is needed and likely to have an effect as planned or, when implemented, to actually have an effect.
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| quasi-experiments | Procedures that resemble those characteristics of true experiments, for example, that some type of intervention or treatment is used and a comparison is provided, but are lacking in the degree of control that is found in true experiments.
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| regression (to the mean) | Because some component of a test score is due to error (as opposed to true score), extreme scores on one test are likely to be closer to the mean on a second test, thus posing a threat to the validity of an experiment in which extreme groups are selected; the amount of this statistical regression is greater for less reliable tests.
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| selection | Selection is a threat to internal validity when, from the outset of a study, differences exist between the kinds of individuals in one group and those in another group in the experiment.
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| simple interrupted time-series design | Quasi-experimental procedure in which changes in a dependent variable are observed for some period of time both before and after a treatment is introduced.
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| subject mortality (attrition) | A threat to internal validity occurs when participants are lost from an experiment, for example, when participants drop out of the research project. The loss of participants changes the nature of a group from that established prior to the introduction of the treatment—for example, by destroying the equivalence of groups that had been established through random assignment.
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| testing | Taking a test generally has an effect on subsequent testing. Testing can threaten internal validity if the effect of a treatment cannot be separated from the effect of testing.
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| threats to internal validity | Possible causes of a phenomenon that must be controlled so a clear cause-and-effect inference can be made.
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| time series with nonequivalent control group design | Quasi-experimental procedure that improves on the validity of a simple time-series design by including a nonequivalent control group; both treatment and comparison groups are observed for a period of time both before and after the treatment.
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