| block randomization | The most common technique for carrying out random assignment in the random groups design; each block includes a random order of the conditions and there are as many blocks as there are subjects in each condition of the experiment.
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| confounding | Occurs when the independent variable of interest systematically covaries with a second, unintended independent variable.
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| demand characteristics | Cues and other information used by participants to guide their behavior in a psychological study, often leading participants to do what they believe the observer (experimenter) expects them to do.
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| double-blind | Both the participant and the observer are kept unaware (blind) to what treatment is being administered.
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| effect size | Index of the strength of the relationship between the independent variable and dependent variable that is independent of sample size.
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| experimenter effects | Experimenters’ expectations that may lead them to treat subjects differently in different groups or to record data in a biased manner.
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| external validity | The extent to which the results of a research study can be generalized to different populations, settings, and conditions.
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| independent groups design | Each separate group of subjects in the experiment represents a different condition as defined by the level of the independent variable.
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| internal validity | Degree to which differences in performance can be attributed unambiguously to an effect of an independent variable, as opposed to an effect of some other (uncontrolled) variable; an internally valid study is free of confounds.
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| matched groups design | Type of independent groups design in which the researcher forms comparable groups by matching subjects on a pretest task and then randomly assigning the members of these matched sets of subjects to the conditions of the experiment.
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| mechanical subject loss | Occurs when a subject fails to complete the experiment because of equipment failure or because of experimenter error.
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| meta-analysis | Analysis of results of several (often, very many) independent experiments investigating the same research area; the measure used in a meta-analysis is typically effect size.
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| natural groups designs | Type of independent groups design in which the conditions represent the selected levels of a naturally occurring independent variable, for example, the individual differences variable age.
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| placebo control | Procedure by which a substance that resembles a drug or other active substance but that is actually an inert, or inactive substance is given to participants.
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| random assignment | Most common technique for forming groups as part of an independent groups design; the goal is to establish equivalent groups by balancing individual differences.
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| random groups design | Most common type of independent groups design in which subjects are randomly selected or randomly assigned to each group such that groups are considered comparable at the start of the experiment.
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| replication | Repeating the exact procedures used in an experiment to determine whether the same results are obtained.
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| selective subject loss | Occurs when subjects are lost differentially across the conditions of the experiment as the result of some characteristic of each subject that is related to the outcome of the study.
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| statistically significant | When the probability of an obtained difference in an experiment is smaller than would be expected if error variation alone were assumed to be responsible for the difference, the difference is statistically significant.
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