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318 PART IV: Applied Researchthat are peculiar to one group’s situation, an event may have more of an impacton that group than on another. Consider, for example, research involving aninvestigation of the effectiveness of an AIDS awareness campaign involvingtwo college campuses (one treatment and one control). Nationwide media attentionto AIDS might reasonably be assumed to affect students on both campusesequally. However, if a student with AIDS died at one college during thestudy and the story was featured in the college newspaper, we would assumethat research participants at this student’s college would be affected differentlycompared to those at the other. In terms of assessing the effect of an AIDSawareness campaign, this situation would represent an additive effect of selectionand history.Finally, an additive effect of selection and instrumentation might occur if a testinstrument is relatively more sensitive to changes in one group’s performancethan to changes in another’s. This occurs, for instance, when ceiling or flooreffects are present. Such is the case when a group scores initially so low on aninstrument (floor effect), that any further drop in scores cannot be reliably measured,or so high (ceiling effect) that any more gain cannot be assessed. As youcan imagine, a threat to internal validity would be present if an experimentalgroup showed relatively no change (due to floor or ceiling effects), while a controlgroup changed reliably because its mean performance was initially near themiddle of the measurement scale.One of the great advantages of true experiments is that they control for allthese threats to internal validity. As Campbell (1969) emphasizes, true experimentsshould be conducted when possible, but if they are not feasible, quasiexperimentsshould be conducted. “We must do the best we can with what isavailable to us” (p. 411). Quasi-experiments represent the best available compromisebetween the general aim of gaining valid knowledge regarding theeffectiveness of a treatment and the realization that true experiments are notalways possible.Problems That Even True Experiments May Not Control• Threats to internal validity that can occur in any study includecontamination, experimenter expectancy effects, and novelty effects.• Contamination occurs when information about the experiment iscommunicated between groups of participants, which may lead toresentment, rivalry, or diffusion of treatment.• Novelty effects occur when people’s behavior changes simply because aninnovation (e.g., a treatment) produces excitement, energy, and enthusiasm.• Threats to external validity occur when treatment effects may not begeneralized beyond the particular people, setting, treatment, and outcomeof the experiment.Before considering specific quasi-experimental procedures, we should pointout that even true experiments may not control for all possible threats to theinterpretation of an experimental outcome. Although major threats to internalvalidity are eliminated by the true experiment, there are some additional threatsthat the investigator who is working in natural settings must guard against. We