Siegfried Gabler, Sabine Häder

Combining Landline Phone Samples and Mobile Phone Samples in Germany:

a Dual Frame Approach

First Outcomes of a DFG*-Project

Siegfried Gabler, Sabine Häder

*DFG = Deutsche Forschungsgemeinschaft = German Research Foundation

Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Overview

• The main problem: The cell phone only households

• Description of the DFG-Project

• First results: - sampling procedure - weighting


Quelle: http://www.bitkom.org/51940_50446.aspx


A challenge for survey methodology:The 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


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


Sampling procedure for cell phone numbers by BIK ASCHPURWIS + BEHRENS GmbH

All hypothetical numbers assigned:279,000,000 numbers*

Generated numbers on basis of used

number blocks :

178,050,000 numbers

Registered numbers2,297,460

(January 2008)

*) Provider O2 allocates eight-digit numbers in connection with the prefix 0176. even thoughonly seven-digit numbers were ordered at RegTP.


Dual frame survey

Frame of landine phone numbers

Frame of cell phone numbers

household

person

person

Landline Cell phone

n % n %

Exclusive use by myself

790 91.1 1051

90.6

Other people use it from time to time

57 6.6 96 8.3

Share it with others 20 2.3 13 1.1

∑ 867 100.0 1160 100.0

Availability: cell phone usage


Landline MF quantity of numbers in frame mF quantity of numbers in sample Selection: simple random

sample

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

i = 1. ... . N (N is number of target persons in population

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.



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).

Inclusion Probability

1, 1,...,

F CF C

i i iF Ci

m mk k i NM z M

( 1) 1 1 1 1 ( 1)

( 1) ( 1)

1 1 1 1

F F F C F C C CF F F C C F C C

ij i j i j i j i jF F F C F C C Ci j i j

F F F C F C CF F F C F C Ci j i j j i iF F F C F C C

i j i j

m m m m m m m mk k k k k k k k

M M z z M M z M M z M M

m m m m m m mk k k k k k kM M z z M M z M M z M

CCj C

i j

mkM

Horvitz-Thompson-Estimator

0

11 1

Fi

F F Fi i

j FkFi F

j i

F

k M k

j m j

z M

m

1 1 1F C F C F Ci i i i i i i

1

C Ci

CCi C

C

M k

m

M

m


Weighting modelProblem:

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

1i.e.

2.5

Fik


Availability: Quantity of cell phone numbers

Landline 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


Example DFG-Project

1.08F C

F C

M m

m M

Landline: MF = 129,725,800

mF = 16,154

1, 1,...,

F CF C

i i iF Ci

m mk k i NM z M

Cell phone: MC = 178,050,000

mC = 23,955

n = 1,009 + 1,162 = 2,171

in sample in sample

1

1 11.08

1.08

i

i FCii F

j j j j Cij

j

nw n

kk

kzk

z



v4 = variable in data file

Histogram of weights

weights

Fre

quen

cie

s

0 2 4 6 8

0

50

100

150

200

250

Cell phone

Landline


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)


Siegfried Gabler, Sabine Häder

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