| baseline | In a single case design, the subject's behavior during a control period before introduction of the experimental manipulation.
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| control series design | An extension of the interrupted time series quasi-experimental design in which there is a comparison or control group.
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| history effect | As a threat to the internal validity of an experiment, refers to any outside event that is not part of the manipulation that could be responsible for the results.
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| instrument decay | As a threat to internal validity, the possibility that a change in the characteristics of the measurement instrument is responsible for the results.
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| interrupted time series design | A design in which the effectiveness of a treatment is determined by examining a series of measurements made over an extended time period both before and after the treatment is introduced. The treatment is not introduced at a random point in time.
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| maturation effect | As a threat to internal validity, the possibility that any naturally occurring change within the individual is responsible for the results.
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| multiple baseline design | Observing behavior before and after a manipulation under multiple circumstances (across different individuals, different behaviors, or different settings).
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| nonequivalent control group design | A quasi-experimental design in which nonequivalent groups of subjects participate in the different experimental groups, and there is no pretest.
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| nonequivalent control group pretest-posttest design | A quasi-experimental design in which nonequivalent groups are used, but a pretest allows assessment of equivalency and pretest-posttest changes.
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| one group posttest-only design | A quasi-experimental design that has no control group and no pretest comparison; a very poor design in terms of internal validity.
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| one group pretest-posttest design | A quasi-experimental design in which the effect of an independent variable is inferred from the pretest-posttest difference in a single group.
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| program evaluation | Research designed to evaluate programs (e.g., social reforms, innovations) that are designed to produce certain changes or outcomes in a target population.
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| quasi-experimental design | A type of design that approximates the control features of true experiments to infer that a given treatment did have its intended effect.
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| reversal design | A single case design in which the treatment is introduced after a baseline period and then withdrawn during a second baseline period. It may be extended by adding a second introduction of the treatment. Sometimes called a "withdrawal" design.
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| selection differences | Differences in the type of subjects who make up each group in an experimental design; this situation occurs when participants elect which group they are to be assigned to.
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| single case experiment | An experiment in which the effect of the independent variable is assessed using data from a single participant.
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| regression toward the mean | Also called statistical regression; principle that extreme scores on a variable tend to be closer to the mean when a second measurement is made.
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| testing effect | A threat to internal validity in which taking a pretest changes behavior without any effect on the independent variable.
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