Irene wants to examine the effect of a defendant's appearance on the judgment of guilt for a crime. She has participants read an identical account of the crime except for the defendant's appearance. A group of high school students receives the description of an attractive defendant, while a group of senior citizens receives the description of an unattractive defendant. Both groups are then asked to rate the defendant's guilt on a seven-point scale. A major cause for confusion in Irene's experiment is the _____