SSI offers efficient and comprehensive Random Digit (RDD) telephone samples in 23 countries: Australia, Austria, Belgium, Canada, China, Denmark, Finland, France, Germany, Ireland, Italy, Japan, Luxembourg, Netherlands, Norway, Poland, Portugal, Puerto Rico, Spain, Sweden, Switzerland, the United Kingdom, and the United States.
Creation of the Databases
SSI builds a series of databases from which to generate probable working residential phone numbers in proportion to the population. Known directory-listed telephone numbers are drawn from databases periodically provided by a service provider, directory publisher, or a commercial database producer. The databases are not limited to single providers. The databases can be used to identify area codes and active blocks of telephone numbers as a part of the process for creating an RDD database. SSI analyzes the distribution of directory-listed numbers to create its own unique database of known area codes and working blocks of 100 consecutive numbers.
In addition to a database of directory-listed numbers, additional resources are employed to balance the sampling frame in accordance with population as well as to validate, clean, and update the data derived from the listed source. These additional sources include appropriate data obtained from national post offices, telephone numbering agencies and statistical agencies. Data is cross-referenced from these multiple sources, distributions analyzed, and addresses and geographic locations verified. Changes and additions are made at lower levels of geography, usually defined as postal codes.
Selection of the Sample
Sample geographies can be selected on a national basis or by sub-geographies, which vary in accordance with the county but usually include the equivalent of county, post code, Nielsen region, and area code. Once geographies have been determined, all working blocks (all numbers except the last two positions of the subscriber number) are organized in ascending order by county, postal code, area code, and subscriber number. Once the quota has been allocated to the sampling frame, which constitutes the geography selected, a sampling interval is calculated by summing the number of listed residential numbers in each eligible block within the frame, and dividing that sum by the number of sampling points assigned to the frame. From a random start between zero and the sampling interval, blocks are systematically selected in proportion to their density of directory-listed households. Once a block has been selected, a two-digit random number in the range of 00-99 is appended to the working block to form a telephone number of the correct length. These points serve as an element sample representative of the geography under study.
Increase Efficiency with Sample Screening
Following sample generation, SSI can employ Sample Screening, a process that identifies and removes most of the randomly generated, unassigned or unavailable subscriber, fax, and modem numbers. This nets a higher working phones, or obtainable, rate, which brings the sample’s efficiency closer to that of a directory-listed sample.
Online Sampling Available
The SSI-SNAP Online Ordering System allows users to order sample quickly and inexpensively through the Internet.
Telephone Sample
