Lambert and Pinheiro (2016) describe a study in which researchers try to identify characteristics of cell phone calls that suggest the phone is being used fraudulently. For each cell phone call, the researchers recorded information on its direction (incoming or outgoing), location (local or roaming), duration, time of day (morning, afternoon or evening), day of week, and whether or not the call took place on a weekday or weekend.

a. Identify the observational units in this study.

b. Identify the categorical variables mentioned in the paragraph. Indicate which are nominal or ordinal.

c. Identify the quantitative variables mentioned in the preceding paragraph.