Unobtrusive measures are those measures used to collect data that are found naturally in field settings and that can be used without participants' awareness or alteration in the natural course of events. Questionnaires, interviews, and direct observation are intrusive in the sense that the participants realize they [Page 428]are being questioned or watched. A major difficulty with subjects' awareness that they are participants is that their behavior may be affected by this knowledge. The major types of unobtrusive measures include physical traces (such as worn floors), books, computers, documents, letters, and observations in which the subject is unaware of being researched.