Change blindness is an issue of attention and representation. If we fail to represent an object in a scene either before or after a change, then we won’t notice the change, and we tend not to represent objects that aren’t important to the meaning of what we’re looking at, because we’re just not paying attention to them.
Simons and Levin conducted an experiment in which the two experimenters who acted as direction-takers were dressed as construction workers, who are common on college campuses, and generally ignored by students and faculty alike. The rest of the procedure was the same (ask directions, guys with door walk in between them, experimenters switch, and so on). This time, only 4 of 12 young participants (college age) noticed the switch, supporting the hypothesis that people only notice the change when the appearance of the individuals was relevant.