| 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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| 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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| 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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| 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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| 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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| threats to internal validity | Possible causes of a phenomenon that must be controlled so a clear cause-effect inference can be made.
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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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| 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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| 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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| 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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| double-blind | Both the participant and the observer are kept unaware (blind) to what treatment is being administered.
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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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| 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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| Cohen's d | A frequently used measure of effect size in which the difference in means for two conditions is divided by the average variability of participants' scores (within-group standard deviation). Based on Cohen's guidelines, d values of .20, .50, and .80 represent small, medium, and large effects, respectively, of an independent variable.
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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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| Null Hypothesis Significance Testing (NHST) | A procedure for statistical inference used to decide whether a variable has produced an effect in a study. NHST begins with the assumption that the variable has no effect (see null hypothesis), and probability theory is used to determine the probability that the effect (e.g., a mean difference between conditions) observed in a study would occur simply by error variation ("chance"). If the likelihood of the observed effect is small (see level of significance), assuming the null hypothesis is true, we infer the variable produced a reliable effect (see statistically significant).
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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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| confidence intervals | Indicate the range of values which we can expect to contain a population value with a specified degree of confidence (95%).
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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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| individual differences variable | A characteristic or trait that varies consistently across individuals, such as level of depression, age, intelligence, gender. Because this variable is formed from pre-existing groups (i.e., it occurs "naturally"), an individual differences variable is sometimes called a natural groups variable. Another term sometimes used synonymously with individual differences variable is subject variable.
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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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