Siegfried Gabler, Sabine Häder
*
DFG = Deutsche Forschungsgemeinschaft
= German Research Foundation

• The main problem:
The cell phone only households
• Description of the DFG-Project
• First results:
- sampling procedure
- weighting
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Quelle: http://www.bitkom.org/51940_50446.aspx
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

A challenge for survey methodology:
The cell phone only households
Slovakia 2008
Finland End of 2006
55%
52%
Poland
France
UK
2007
2006
2006
Germany 2007
24%
17%
10%
8%
Source: Lepkowski et al 2008; own investigation
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

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

Sampling procedure for cell phone numbers by
BIK n ASCHPURWIS + BEHRENS GmbH
All hypothetical numbers assigned:
279,000,000 numbers*
Generated numbers on basis of used number blocks :
178,050,000 numbers
Registered numbers
2,297,460
(January 2008)
*) Provider O2 allocates eight-digit numbers in connection with the prefix 0176. even though only seven-digit numbers were ordered at RegTP.
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Dual frame survey
Frame of landine phone numbers
Frame of cell phone numbers household person person
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Exclusive use by myself
Other people use it from time to time
Share it with others
∑
Landline Cell phone n % n %
790 91.1 1051 90.6
57 6.6 96 8.3
20 2.3 13 1.1
867 100.0 1160 100.0
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

i = 1. ... . N (N is number of target persons in population
M F quantity of numbers in frame m F quantity of numbers in sample
Selection: simple random sample
M C quantity of numbers in frame m C quantity of numbers in sample
Selection: simple random sample k i
C Quantity of cell phone numbers allowing access to person i.
k i
F Quantity of landline numbers allowing access to the household to which person i belongs.
Selection: Kish table/last birthday z i
Size of household to which person i belongs.
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

(not necessarily different)
(Could be a problem in small regional samples).
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

 i
F  k i
F j


0



1 1

1 z i
 i
Inclusion Probability
1

1
 i
F

1
  i
C

  i
F   i
C    i i
C



 k i
F


M
F  k i
F j m
F  j

M F


 m F





 i
C



M
C  k i
C m
C

M
C

 
 m
C 



  i k i
F m
F
M
F z i k i
C m
C
M
C
, i

1,..., N
 ij
 k k
F i j



 k i
F
  i j m
F
M
F m
F
( m
F
M
F
( M
F


1)

1 1
1) z z i j

 k j
F m
F

 
1 1
M
F z z i j
 k k
C i j m m
C
M M
C z i


 k i
F m
F
M
F

 k
C j k k i j m
M
C
C

 z i
F m m
C

1

M M
C z j k k
C i j m
C
( m
C
M
C
( M
C

1)

1)

 k j
F m
F
M
F

 k i
C m
M
C
C



1
 z j

 k i
C m
C
M
C

 k
C j m
C
M
C


Horvitz-Thompson-Estimator
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

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 numbers
ISDN = integrated services digital network i.e.
k i
F
1
2.5
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Availability: Quantity of cell phone numbers
One
Two
Three
Four
Five and more
∑ missings
Landline n %
771 88.82
71
19
3
4
8.18
2.19
0.35
0.46
Cell phone n %
915 78.74
192 16.52
34 2.93
9
12
0.78
1.03
868 100.0
1,162 100.0
141 0
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Example DFG-Project
Landline: M F = 129,725,800 m F = 16,154
Cell phone: M C = 178,050,000 m C = 23,955
 i
 k i
F m
F
F
M z i k i
C m
C
M
C
, i

1,..., N
M
F m
C
F m M
C

1.08
w
 n i
1
 i j

in sample

1 j



1.08
 k i
F


 z i k i
C n j

in sample
1
1.08
F
 k j  z j k
C j n = 1,009 + 1,162 = 2,171
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

respondents
2,171 landline phone onlys
141 cell phone
2,030 k F analog
81
= 1; k C = 0 k F
ISDN
59
= 2.5; k C = 0 real landline phone
1,778 no real landline phone
252 analog
717
ISDN
1,061 virtual landline phone k F
77
= 1; k C = v4 cell phone onlys k F
175
= 0; k C = v4 virtual landline phone k F
93
= 2; k C = v4 v4 = variable in data file no virtual landline phone k F
624
= 1; k C = v4 virtual landline phone 153 k F = 3.5; k C = v4 no virtual landline phone k F
908
= 2.5; k C = v4
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

Histogram of weights
250
200
150
100
Cell phone
Landline
50
0
0 2 4 weights
6
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008
8

• 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 DFGprogram „Survey Methodology“: Mobilpanel together with Marek Fuchs (University of Kassel)
Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008