In a suburban community in the Southeast, neighborhood security guards have their residents’ permission to call local police if anyone is observed entering the home of a resident who is out of town. When residents leave town, they report their departure and return dates to the neighborhood security office. If guards observe someone entering the home of a resident who is out of town, they will immediately call the police. The residential burglary rate for that community has decreased since the implementation of this reporting procedure.
If it is true that the burglary rate has decreased in this community since the reporting procedure was implemented, then which of the following would be most important to know to make sure the conclusion is valid?