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Glossary 455theory Logically organized set of propositions that serves to define events, describerelationships among events, and explain the occurrence of these events; scientifictheories guide research and organize empirical knowledge.threats to internal validity Possible causes of a phenomenon that must be controlledso a clear cause-effect inference can be made.time sampling Selection of observation intervals either systematically or randomlywith the goal of obtaining a representative sample of behavior.time series with nonequivalent control group design (See also simple interruptedtime- series design.) Quasi-experimental procedure that improves on the validity ofa simple time- series design by including a nonequivalent control group; both treatmentand comparison groups are observed for a period of time both before and afterthe treatment.t-test for independent groups An inferential test for comparing two means from differentgroups of subjects.Type I error The probability of rejecting the null hypothesis when it is true, equal tothe level of significance, or alpha.Type II error The probability of failing to reject the null hypothesis when it is false.unobtrusive (nonreactive) measures Measures of behavior that eliminate the problemof reactivity because observations are made in such a way that the presence of theobserver is not detected by those being observed.validity The “truthfulness” of a measure; a valid measure is one that measures whatit claims to measure.variability See measures of dispersion.