Random digit dialing
The term random digit dialing ( RDD ) or random telephone survey describes a procedure for drawing samples in telephone surveys . Parts of the telephone numbers are generated randomly. The method was developed in the USA, where the first three digits of the ten-digit telephone numbers stand for a region and the next three for an exchange. These combinations are entered in lists and are available. RDD then appends the last four digits at random.
One reason for using RDD techniques is that fewer and fewer telephone connections (landline or mobile) are listed in the phone book these days. A variant of random digit dialing is the randomized last digit method .