Contents

450 Glossaryinformed consent Explicitly expressed willingness to participate in a research projectbased on clear understanding of the nature of the research, of the consequences ofnot participating, and of all factors that might be expected to influence willingness toparticipate.instrumentation Changes over time can take place not only in the participants of anexperiment, but also in the instruments used to measure the participants’ performance.These changes due to instrumentation can threaten internal validity if theycannot be separated from the effect of the treatment.interaction effect When the effect of one independent variable differs depending onthe level of a second independent variable.internal validity Degree to which differences in performance can be attributed unambiguouslyto an effect of an independent variable, as opposed to an effect of someother (uncontrolled) variable; an internally valid study is free of confounds.interobserver reliability Degree to which two independent observers are in agreement.interrupted time-series design See simple interrupted time-series design and timeseries with nonequivalent control group design.interviewer bias Occurs when the interviewer tries to adjust the wording of a questionto fit the respondent or records only selected portions of the respondent’s answers.level of significance The probability when testing the null hypothesis that is used toindicate whether an outcome is statistically significant. Level of significance, or alpha,is equal to the probability of a Type I error.linear trend A trend in the data that is appropriately summarized by a straight line.longitudinal design Research design in which the same sample of respondents isinterviewed (surveyed) more than once.main effect Overall effect of an independent variable in a complex design.matched groups design Type of independent groups design in which the researcherforms comparable groups by matching subjects on a pretest task and then randomlyassigns the members of these matched sets of subjects to the conditions of theexperiment.maturation Change associated with the passage of time per se is called maturation.Changes participants undergo in an experiment that are due to maturation and notdue to the treatment can threaten internal validity.mean The arithmetic mean, or average, is determined by dividing the sum of the scoresby the number of scores contributing to that sum. The mean is the most commonlyused measure of central tendency.measurement scale One of four levels of physical and psychological measurement:nominal (categorizing), ordinal (ranking), interval (specifying distance betweenstimuli), and ratio (having an absolute zero point).measures of central tendency Measures such as the mean, median, and mode thatidentify a score that the data tend to center around.measures of dispersion (variability) Measures such as the range and standard deviationthat describe the degree of dispersion of numbers in a distribution.mechanical subject loss Occurs when a subject fails to complete the experimentbecause of equipment failure or because of experimenter error.median The middle point in a distribution, above which half the scores fall and belowwhich half fall.meta-analysis Analysis of results of several (often, very many) independent experimentsinvestigating the same research area; the measure used in a meta-analysis istypically effect size.minimal risk A research participant is said to experience minimal risk when probabilityand magnitude of harm or discomfort anticipated in the research is not greater