Contents

CHAPTER 6: Independent Groups Designs 195effects of the event on participants’ performance will be equivalent, or averaged,across the conditions. Block randomization also works to balance othertime-related variables, such as changes in experimenters or even changesin the populations from which subjects are drawn. For example, a perfectlyacceptable experiment could be done drawing students from both fall andspring semester classes if a block randomization schedule is used. The beautyof block randomization is that it will balance (or average) any characteristicsof participants (including the effects of time-related factors) across the conditionsof an experiment.If you want to practice the procedure of block randomization, you can doChallenge Question 1A at the end of this chapter.Key ConceptThreats to Internal Validity• Randomly assigning intact groups to different conditions of theindependent variable creates a potential confounding due to preexistingdifferences among participants in the intact groups.• Block randomization increases internal validity by balancing extraneousvariables across conditions of the independent variable.• Selective subject loss, but not mechanical subject loss, threatens the internalvalidity of an experiment.• Placebo control groups are used to control for the problem of demandcharacteristics, and double-blind experiments control both demandcharacteristics and experimenter effects.We’ve seen that internal validity is the degree to which differences in performanceon a dependent variable can be attributed clearly and unambiguouslyto an effect of an independent variable, as opposed to some otheruncontrolled variable. These uncontrolled variables are often referred to asthreats to internal validity. These threats are potential alternative explanationsfor a study’s findings. In order to make a clear cause-and-effectinference about an independent variable, threats to internal validity mustbe controlled. We next describe several problems in experimental researchthat can result in threats to internal validity, and methods to control thesethreats.Testing Intact Groups Random assignment is used to form comparable groupsin the random groups design. There are times, however, when noncomparablegroups are formed even when random assignment appears to have been used.This problem occurs when intact groups (not individuals) are randomly assignedto the conditions of an experiment. Intact groups are formed prior tothe start of the experiment. For example, the different sections of an introductorypsychology course are intact groups. Students are not randomly assignedto different sections of introductory psychology (although sometimes schedulingclasses seems random!). Students often choose to be in a particular sectionbecause of the time the class meets, the instructor, friends who will be in theclass, and any number of other factors. If a researcher were to randomly assign