Combining Landline Phone Samples and Mobile Phone Samples in Germany: a Dual Frame Approach First Outcomes of a DFG*-Project

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Combining Landline Phone Samples and Mobile Phone
Samples in Germany a Dual Frame ApproachFirst
Outcomes of a DFG-Project

• Siegfried Gabler, Sabine Häder

DFG Deutsche Forschungsgemeinschaft
German Research Foundation
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Overview

• The main problem
• The cell phone only households
• Description of the DFG-Project
• First results
• - sampling procedure
• - weighting

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Quelle http//www.bitkom.org/51940_50446.aspx
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A challenge for survey methodologyThe cell
phone only households
Slovakia 2008 55
Finland End of 2006 52
Poland 2007 24
France 2006 17
UK 2006 10
Germany 2007 8
Source Lepkowski et al
2008 own investigation
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DFG-Project Interviews via cell phones

• Cooperation TU Dresden and GESIS-ZUMA
• Two parallel surveys one via landline, one via
  cell phone
• Aims of the project- Investigation of mode
  effects- Response behaviour- Weighting model
  for the combination of both samples
• Respectively ca. 1,000 people interviewed
  Realization Landline 1,009
  Cell phones 1,161
• Survey period October 2007-April 2008

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Sampling procedure for cell phone numbers by
BIK n ASCHPURWIS BEHRENS GmbH
) Provider O2 allocates eight-digit
numbers in connection with the prefix 0176. even
though only seven-digit numbers were ordered at
RegTP.
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Dual frame survey
Frame of landine phone numbers
Frame of cell phone numbers
person
household
person
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Availability cell phone usage
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i 1. ... . N (N is number of target persons in
population
Landline MF quantity of numbers in frame mF
quantity of numbers in sample Selection
simple random sample
Cell phone MC quantity of numbers in frame mC
quantity of numbers in sample Selection
simple random sample
kiC Quantity of cell phone numbers allowing
access to person i. kiF Quantity of landline
numbers allowing access to the household to which
person i belongs. Selection Kish
table/last birthday zi Size of household to
which person i belongs.
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Weigthing model basic assumption
The probability of choosing two (not necessarily
different) members of the same household via
different modes is insignificant.
(Could be a problem in small regional samples).

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Inclusion Probability
Horvitz-Thompson-Estimator
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• Weighting model
• Problem
• Most of the household members do not know
  the exact quantity of landline numbers allowing
  access to their household.
• Therefore the simplified assumption

Landline (analog) 1 number
Landline (ISDN) 2.5 numbersISDN integrated
services digital network
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Availability Quantity of cell phone numbers
Landline Landline Cell phone Cell phone
  n n
One 771 88.82 915 78.74
Two 71 8.18 192 16.52
Three 19 2.19 34 2.93
Four 3 0.35 9 0.78
Five and more 4 0.46 12 1.03
? 868 100.0 1,162 100.0
missings 141 0
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Example DFG-Project
Landline MF 129,725,800 mF 16,154
Cell phone MC 178,050,000
mC 23,955
n 1,009 1,162 2,171
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v4 variable in data file
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Following Steps

• Evaluation
• Publication of the results in Häder. M. und
  Häder. S. (Hrsg.) Telefonbefragungen über das
  Mobilfunknetz. VS Verlag. Winter 2008.
• Consecutive application for the DFG calculation
  of the response rates
• Conference in Mannheim about the results of the
  DFG-Project. November 14th. 2008
• Further project within the framework of the
  DFG-program Survey Methodology Mobilpanel
  together with Marek Fuchs (University of Kassel)