The ADM-Sampling-System for Telephone Surveys
In Germany, no directory is available to market research agencies that contains the
addresses of all private households or individuals, nor is there a directory of all the telephone
numbers used by private households. Since even public telephone books are incomplete,
there is no official source on which to base telephone surveys. Furthermore, no official
statistical data is available describing this population adequately for a sampling frame.
The “Arbeitsgemeinschaft ADM-Telefonstichproben” (a group of market research agencies
responsible for the sampling system) has closed this gap by providing a sampling frame to
member agencies. This frame allows telephone samples to be selected representatively with
respect to all households with a telephone in Germany and to the persons living in those
households. Furthermore, the sampling system contains the statistical data which is
necessary to fully describe the population described above.
The sampling frame itself – the so-called “ADM-Telefonstichproben-System” – is based on
the range of numbers available in the German telephone network as updated, monitored and
published on the Internet by the “Bundesnetzagentur” (the governmental agency for the
German telephone network). This range of numbers covers all possible telephone numbers
in Germany, whether actually in use or not. Based on methods developed by Sabine Häder
and Siegfried Gabler to generate telephone numbers1), the “Bundesnetzagentur” data are
used according to the following two rules:
•
Numbers from the German landline-based telephone network are generated on the
basis of blocks of numbers with a range of 10 (so-called “Festnetzdatei”)
•
Numbers from the German cellular telephone network are generated on the basis of
number blocks with a range of 10000 (so-called “Mobilfunkdatei”).
As almost 100% of the households in Germany have a telephone and much more than 90%
use at least one landline-based phone, representative telephone surveys of the German
population can be conducted.
Using the landline-based sampling frame has a further advantage. Due to the fact that the
vast majority of these telephone numbers are listed with the owner's full address, the
numbers in this sampling frame include regional markers. These allow stratified sampling
procedures to be used, differentiating down to the community level or even to the district
level in larger cities. Therefore the well-known, proven stratification method of face-to-facesurveys can be used here too.
1) Cf. S. Gabler, S. Häder; Ein neues Stichprobendesign für telefonische Umfragen in Deutschland;
in: S. Gabler, S. Häder, J. H. P. Hoffmeyer-Zlotnik (Hrsg.); Telefonstichproben in Deutschland,
S. 69 – 88, Opladen 1998
2) Cf. L. H. Cox; A constructive procedure for unbiased controlled rounding; in: Journal of the
American Statistical Association 82, S.520 – 524, 1987
Due to the very small proportion of listed cellular phone numbers to non-listed numbers, the
second sampling frame cannot be stratified according to regional strata. However since
every provider of cellular telephone services has their own prefixes, this information can be
used as a stratification criterion to improve the sampling design.
To best utilize this stratification, sampling is done using the method for random allocation
developed by L. H. Cox.2) A major advantage of this method is that it leads to stratified
samples without any accumulation of rounding effects.
As the sampling of the telephone sample, as well as the selection of the target persons within
the households, is done randomly, the entire telephone survey is based on random sampling.
Therefore surveys based on the ADM-Telephone-Sampling-System fully meet the scientific
requirements regarding randomization based on statistical methods.
Christian von der Heyde, Spokesperson of ADM Sampling Systems, July 2009
1) Cf. S. Gabler, S. Häder; Ein neues Stichprobendesign für telefonische Umfragen in Deutschland;
in: S. Gabler, S. Häder, J. H. P. Hoffmeyer-Zlotnik (Hrsg.); Telefonstichproben in Deutschland,
S. 69 – 88, Opladen 1998
2) Cf. L. H. Cox; A constructive procedure for unbiased controlled rounding; in: Journal of the
American Statistical Association 82, S.520 – 524, 1987