| Comparison group | The group in a research study that receives a different treatment from that of the experimental group.
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| Control | Efforts on the part of the researcher to remove the effects of any variable other than the independent variable that might affect performance on a dependent variable.
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| Control group | The group in a research study that is treated "as usual."
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| Counterbalanced designs | Designs in which all groups receive all treatments. Each group receives the treatments in a different order, and all groups are posttested after each treatment.
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| Criterion variable | The variable that is predicted in a prediction study; also any variables used to assess the criterion-related validity of an instrument.
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| Dependent variable | A variable affected or expected to be affected by the independent variable; also called "criterion" or "outcome variable."
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| Experiment | A research study in which one or more independent variables is systematically varied by the researcher to determine the effects of this variation.
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| Experimental group | The group in a research study that receives the treatment (or method) of special interest in the study.
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| Experimental research | Research in which at least one independent variable is manipulated, other relevant variables are controlled, and the effect on one or more dependent variables is observed.
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| Experimental variable | The variable that is manipulated (systematically altered) in an intervention study by the researcher.
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| Extraneous variables | Variables that make possible an alternative explanation of results; an uncontrolled variable.
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| Factorial design | An experimental design that involves two or more independent variables (at least one of which is manipulated) in order to study the effects of the variables individually, and in interaction with each other, upon a dependent variable.
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| Gain score | The difference between the pretest and posttest scores of a measure.
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| Independent variable | A variable that affects (or is presumed to affect) the dependent variable under study and is included in the research design so that its effect can be determined; sometimes called the "experimental" or "treatment" variable.
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| Interaction | An effect created by unique combinations of two or more independent variables; systematically evaluated in a factorial design.
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| Matching design | Consists of two groups of items listed in columns. Respondents are required to match the item in the left column that corresponds most closely with an item in the right column.
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| Mechanical matching | A process of pairing two persons whose scores on a particular variable are similar.
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| Moderator variables | Variables that may or may not be controlled but has an effect on the research situation.
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| Nonequivalent control group design | An experimental design involving at least two groups, both of which may be pretested; one group receives the experimental treatment, and both groups are posttested. Individuals are not randomly assigned to treatments.
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| One-group pretest-posttest design | A weak experimental design involving one group that is pretested, exposed to a treatment, and posttested.
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| One-shot case study design | A weak experimental design involving one group that is exposed to a treatment and then posttested.
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| Outcome variable | See dependent variable.
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| Pretest treatment interaction | The possibility that subjects may respond or react differently to a treatment because they have been pretested, thereby creating a threat to internal validity.
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| Quasi-experimental designs | A type of experimental design in which the researcher does not use random assignment of subjects to groups.
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| Random assignment | The process of assigning individuals or groups randomly to different treatment conditions.
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| Randomized posttest-only control group design | An experimental design involving at least two randomly formed groups; one group receives a treatment, and both groups are posttested.
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| Randomized pretest-posttest control group design | An experimental design that involves at least two groups; both groups are pretested, one group receives a treatment, and both groups are posttested. For effective control of extraneous variables, the groups should be randomly formed.
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| Randomized Solomon four-group design | An experimental design that involves random assignment of subjects to each of four groups. Two groups are pretested, two are not, one of the pretested groups and one of the unpretested groups receive the experimental treatment, and all four groups are posttested.
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| Random selection | The process of selecting a random sample.
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| Regressed gain score | A score indicating amount of change that is determined by the correlation between scores on a posttest and a pretest (and/or other scores). It provides more stable information than a simple posttest-pretest difference.
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| Static-group comparison design | A weak experimental design that involves at least two nonequivalent groups; one receives a treatment and both are posttested.
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| Static-group pretest-posttest design | The same as the static-group comparison design, except that both groups are pretested.
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| Statistical equations | A means of equating groups using statistical prediction.
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| Statistical matching | A means of equating groups using statistical prediction.
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| Time-series design | An experimental design involving one group that is repeatedly pretested, exposed to an experimental treatment, and repeatedly posttested.
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| Treatment variable | see experimental variable.
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