1. 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

2. 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

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

4. Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008 A challenge for survey methodology: The cell phone only households Slovakia200855% FinlandEnd of 200652% Poland200724% France200617% UK200610% Germany20078% Source: Lepkowski et al 2008; own investigation

5. 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 phones1,161 Survey period October 2007-April 2008

6. Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 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 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.

7. 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

8. Availability: cell phone usage Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

9. Landline M F quantity of numbers in frame m F quantity of numbers in sample Selection: simple random sample Landline M F quantity of numbers in frame m F quantity of numbers in sample Selection: simple random sample Cell phone M C quantity of numbers in frame m C quantity of numbers in sample Selection: simple random sample i = 1..... N (N is number of target persons in population 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

10. 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).

11. Inclusion Probability Horvitz-Thompson-Estimator Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

12. 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 1 i.e. 2.5     F i k Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

13. Availability: Quantity of cell phone numbers LandlineCell phone n %n % One77188.8291578.74 Two718.1819216.52 Three192.19342.93 Four30.3590.78 Five and more 40.46121.03 ∑868100.01,162100.0 missings1410 Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

14. Example DFG-Project Landline: M F = 129,725,800 m F = 16,154 Cell phone: M C = 178,050,000 m C = 23,955 n = 1,009 + 1,162 = 2,171 Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

15. respondents 2,171 landline phone onlys 141 analog 81 k F = 1; k C = 0 ISDN 59 k F = 2.5; k C = 0 cell phone 2,030 real landline phone 1,778 analog 717 virtual landline phone 93 k F = 2; k C = v4 no virtual landline phone 624 k F = 1; k C = v4 ISDN 1,061 virtual landline phone153 k F = 3.5; k C = v4 no virtual landline phone 908 k F = 2.5; k C = v4 no real landline phone 252 virtual landline phone 77 k F = 1; k C = v4 cell phone onlys 175 k F = 0; k C = v4 Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008 v4 = variable in data file

16. Histogram of weights weights Frequencies 02468 0 50 100 150 200 250 Cell phone Landline Gabler/Häder, Landline/cell phone, Q2008 Rome 10 July 2008

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