| analysis of covariance | A statistical technique to control for the correlation between a subject variable and a dependent variable in an experiment. This procedure removes the error variance that results from the fact that variability in scores on the dependent variable is due in part to the effect of the subject variable.
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| confounding variable | A variable that is not controlled in a research investigation. In an experiment, the experimental groups differ on both the independent variable and the confounding variable.
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| counterbalancing | A method of controlling for order effects in a repeated measures design by either including all orders of treatment presentation or randomly determining the order for each subject.
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| independent groups design | An experiment in which different subjects are assigned to each group. Also called between-subjects design.
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| internal validity | The certainty with which results of an experiment can be attributed to the manipulation of the independent variable rather than to some other, confounding variable.
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| Latin square | A technique to control for order effects without having all possible orders.
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| matched pairs design | A method of assigning subjects to groups in which pairs of subjects are first matched on some characteristic and then individually assigned randomly to groups.
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| mortality | The loss of subjects who decide to leave an experiment. Mortality is a threat to internal validitywhenthemortality rate is related to the nature of the experimental manipulation.
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| order effect | In a repeated measures design, the effect that the order of introducing treatment has on the dependent variable.
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| posttest-only design | A true experimental design in which the dependent variable (posttest) is measured only once, after manipulation of the independent variable.
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| pretest-posttest design | A true experimental design in which the dependent variable is measured both before (pretest) and after (posttest) manipulation of the independent variable.
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| repeated measures design | An experiment in which the same subjects are assigned to each group. Also called within-subjects 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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