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330 PART IV: Applied ResearchDifferential Statistical Regression The final threat to internal validity that is notcontrolled in the nonequivalent control group design is differential statisticalregression (Shadish et al., 2002). As we described earlier, regression toward themean is to be expected when individuals are selected on the basis of extremescores (e.g., the poorest readers, the workers with the lowest productivity,the patients with the most severe problems). Differential regression can occurwhen regression is more likely in one group than in another. For example, considera nonequivalent control group design in which the participants with themost serious problems are placed in the treatment group. It is possible, evenlikely, that regression would occur for this group. The changes from pretestto posttest may be mistakenly interpreted as a treatment effect if regressionis more likely in the treatment group than in the control group. Because thegroups in the Langer and Rodin study came from the same population andthere is no evidence that one group’s pretest scores were more extreme thananother’s, a threat to internal validity due to differential statistical regression isnot plausible in their study.Expectancy Effects, Contamination, and Novelty Effects Langer and Rodin’sstudy could also have been influenced by three additional threats to internalvalidity that can even affect true experiments—expectancy effects, contamination,and novelty effects. If observers in their study had been aware of theresearch hypothesis, it is possible that they inadvertently might have ratedresidents as being better after the responsibility instructions than before. Thisobserver bias, or expectancy effect, appears to have been controlled, however,because all the observers were kept unaware of the research hypothesis.Langer and Rodin were also aware of possible contamination effects. Residentsin the control group might have become demoralized if they learnedthat residents on another floor were given more opportunity to make decisions.In this case, the use of different floors of the nursing home was advantageous;Langer and Rodin (1976) indicate that “there was not a great deal ofcommunication between floors” (p. 193). Thus, contamination effects do notseem to be present, at least on a scale that would destroy the internal validityof the study.Novelty effects would be present in the Langer and Rodin study if residentson the treatment floor gained enthusiasm and energy as a result of the innovativeresponsibility-inducing treatment. Thus, this new enthusiasm, ratherthan treatment residents’ increased responsibility, may explain any treatmenteffects. In addition, the special attention given the treatment group may haveproduced a Hawthorne effect in which residents on the treated floor felt betterabout themselves. It is difficult to rule out completely novelty effects or aHawthorne effect in this study. According to the authors, however, “There wasno difference in the amount of attention paid to the two groups” (p. 194). Infact, communications to both groups stressed that the staff cared for them andwanted them “to be happy.” Thus, without additional evidence to the contrary,we can conclude that the changes in behavior Langer and Rodin observed weredue to the effect of the independent variable, not to the effect of an extraneousvariable that the investigators failed to control.