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Independent Variable

A circumstance having two or more levels manipulated by the experimenter so that the effects on the dependent variable can be observed.

Dependent Variable

The behavior the experimenter chooses to measure; this behavior may be dependent upon the levels of the independent variable.

Hypothesis

A statement about a predicted relationship between two or more variables.

Non-Directional Hypothesis

A tentative prediction that the levels of an independent variable will cause a change in the dependent variable, but not predicting the direction of that change.

Directional Hypothesis

A tentative prediction that the levels of an independent variable will cause a dependent variable to change in a particular direction.

Control Variables

A circumstance of the experiment that the experimenter sets at a particular level and prevents from varying.

External Validity

The generalizability of an experimental result to a particular real world population, situation, or setting different from that represented in the experiment.

Random Variable

A circumstance in an experiment whose level is determined by chance rather than being controlled by the experimenter.

Random Selection

The use of a random process to choose a sample of items or people from a population.

Random Assignment

The placement of participants selected from a population into experimental groups using an random process.

Randomization Within Constraints

A method of selection in which items are randomly chosen within the bounds of some selection rule or rules. (fr example, conditions are randomly chosen within the constraint that they be represented an equal number of times).

Randomization Within Blocks

A method of selection in which conditions are randomly assigned to trials, within the constraint that each conditions occurs an equal number of times within each block of trials.

Confounding Variable

A variable whose levels are correlated with the levels of the independent variable such that any change in behavior could be due either to the levels of the independent variable or those of the confounding variable.

Internal Validity

The certainty of the assertion that it was the manipulation of the independent variable that caused the change in the dependent variable.

Maturation As A Threat To Internal Validity

A change in the dependent variable due to the participants' aging or becoming more experienced between the administration of the levels of the independent variable.

Selection As A Threat To Internal Validity

A difference in the dependent variable due to any difference in the composition of participant groups exposed to different levels of the independent variable.

Mortality As A Threat To Internal Validity

A difference in the dependent variable due the differential participant attrition from groups exposed to different levels of the independent variable.

Statistical Regression As A Threat To Internal Validity

The movement of participants' extreme scores toward the group mean during repeated testing.

Interactions With Selection

A threat to internal validity caused by a validity threat such a s maturation or history interacting with the threat of selection.

Testing As A Threat To Internal Validity

A change in the dependent variable due to participants' prior exposure to the testing instrument or situation.

History As A Threat To Internal Validity

A change in the dependent variable due to the occurrence of an event between the testing of levels of the independent variable.

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