Random digit dialing
Encyclopedia
Random digit dialing is a method for selecting people for involvement in telephone statistical survey
s by generating telephone number
s at random
. Random digit dialing has the advantage that it includes unlisted numbers that would be missed if the numbers were selected from a phone book. In populations where there is a high telephone
-ownership rate, it can be a cost efficient way to get complete coverage of a geographic area.
RDD is widely used for statistical surveys, including election
opinion polling and selection of experimental control groups .
An important consideration in random digit dialing surveys is bias introduced by non-responders. Non-response bias can be a problem if responders differ from non-responders for the measured variables. For example, nonresponders may not have been contacted because they work multiple minimum-wage jobs. Various techniques are used to reduce the non-response rate, such as multiple call attempts, monetary incentives, advance letters, and leaving messages on answering machines, because reducing the non-response rate may directly reduce non-response bias. In addition, when trying to calculate total error estimates, response rate calculations can be imprecise because it can be difficult to determine whether certain telephone numbers are interviewable.
When the desired coverage area matches up closely enough with country code
s and area codes, random digits can be chosen within the desired area codes. In cases where the desired region doesn't match area codes (for instance, electoral district
s), surveys must rely on telephone databases, and must rely on self-reported address information for unlisted numbers. Increasing use of mobile phones (although there are currently techniques which allow infusion of wireless phones into the RDD sampling frame), number portability
, and VoIP have begun to decrease the ability for RDD to target specific areas within a country and achieve complete coverage.
Statistical survey
Survey methodology is the field that studies surveys, that is, the sample of individuals from a population with a view towards making statistical inferences about the population using the sample. Polls about public opinion, such as political beliefs, are reported in the news media in democracies....
s by generating telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
s at random
Random number generation
A random number generator ) is a computational or physical device designed to generate a sequence of numbers or symbols that lack any pattern, i.e. appear random....
. Random digit dialing has the advantage that it includes unlisted numbers that would be missed if the numbers were selected from a phone book. In populations where there is a high telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
-ownership rate, it can be a cost efficient way to get complete coverage of a geographic area.
RDD is widely used for statistical surveys, including election
Election
An election is a formal decision-making process by which a population chooses an individual to hold public office. Elections have been the usual mechanism by which modern representative democracy operates since the 17th century. Elections may fill offices in the legislature, sometimes in the...
opinion polling and selection of experimental control groups .
An important consideration in random digit dialing surveys is bias introduced by non-responders. Non-response bias can be a problem if responders differ from non-responders for the measured variables. For example, nonresponders may not have been contacted because they work multiple minimum-wage jobs. Various techniques are used to reduce the non-response rate, such as multiple call attempts, monetary incentives, advance letters, and leaving messages on answering machines, because reducing the non-response rate may directly reduce non-response bias. In addition, when trying to calculate total error estimates, response rate calculations can be imprecise because it can be difficult to determine whether certain telephone numbers are interviewable.
When the desired coverage area matches up closely enough with country code
Country code
Country codes are short alphabetic or numeric geographical codes developed to represent countries and dependent areas, for use in data processing and communications. Several different systems have been developed to do this. The best known of these is ISO 3166-1...
s and area codes, random digits can be chosen within the desired area codes. In cases where the desired region doesn't match area codes (for instance, electoral district
Electoral district
An electoral district is a distinct territorial subdivision for holding a separate election for one or more seats in a legislative body...
s), surveys must rely on telephone databases, and must rely on self-reported address information for unlisted numbers. Increasing use of mobile phones (although there are currently techniques which allow infusion of wireless phones into the RDD sampling frame), number portability
Local number portability
Local number portability for fixed lines, and full mobile number portability for mobile phone lines, refers to the ability to transfer either an existing fixed-line or mobile telephone number assigned by a local exchange carrier and reassign it to another carrier...
, and VoIP have begun to decrease the ability for RDD to target specific areas within a country and achieve complete coverage.