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CHAPTER 10: Quasi-Experimental Designs and Program Evaluation 319BOX 10.2EXPERIMENTAL CONTAMINATIONThere are several possible effects resulting fromcommunication between groups of experimentalparticipants. These include (1) resentment on thepart of individuals receiving less desirable treatments,(2) rivalry among groups receiving differenttreatments, and (3) a general diffusion of treatmentsacross the groups (see Cook & Campbell,1979; Shadish et al., 2002).• Resentment Consider a situation in which individualshave been randomly assigned to a control group.Further, assume that control group participants learnthat “other” participants are receiving a beneficialtreatment. What do you think might be the reactionof the control participants? One possibility is that thecontrol participants will feel resentful and demoralized.As Cook and Campbell explain, in an industrial settingthe person receiving the less desirable treatment mayretaliate by lowering productivity. In an educationalsetting, teachers or students might “lose heart” or becomeangry. This effect of “leaked” information abouta treatment may make a treatment look better than itordinarily would because of the lowered performanceof the control group that responds with resentment.• Rivalry Another possible effect that may occur whena control group learns about another group’s goodfortune is a spirit of competition or rivalry. That is, acontrol group might become motivated to reduce theexpected difference between itself and the treatmentgroup. As Cook and Campbell point out, this may belikely when intact groups (such as departments, workcrews, branch offices, and the like) are assigned tovarious conditions. Realizing that another group willlook better depending on how much it distinguishesitself from the control group, participants comprisingthe control group may be motivated to “try harder” soas not to look bad by comparison.• Diffusion of treatments Yet another possible effectof contamination is diffusion of treatments. Accordingto Cook and Campbell, this occurs when participantsin a control group use information given to others tohelp them change their own behavior. For example,control participants may use the information givento participants in the treatment group to imitate thebehavior of individuals who were given the treatment.Of course, this reduces the differences between thetreated and untreated groups and affects the internalvalidity of the experiment.Key ConceptKey Conceptwill use the term contamination to describe one general class of threats to internalvalidity. Contamination occurs when there is communication of informationabout the experiment between groups of participants. Box 10.2 describes theseveral unwanted effects that can occur with contamination.True experiments can also be affected by threats due to experimenter expectancyeffects that occur when an experimenter unintentionally influences the results.Observer bias occurs when researchers’ biases and expectancies lead to systematicerrors in observing, identifying, recording, and interpreting behavior.( Various ways to control observer or experimenter effects were outlined inChapter 4 and Chapter 6, e.g., using a double-blind procedure.)Novelty effects can occur when an innovation, such as an experimentaltreatment, is introduced (Shadish et al., 2002). For example, if little in the wayof change or innovation has occurred for some time at a work site, employeesmay become excited or energized by the novelty (or newness) of their workenvironment when an intervention is introduced. Employees’ newfound enthusiasm,rather than the intervention itself, may account for the “success” of theintervention. The opposite of a novelty effect can occur as a disruption effect, inwhich an innovation, perhaps with new work procedures, disrupts employees’work to such an extent that they cannot maintain their typical effectiveness.