Selectivity in the German Mobility Panel

1. Selectivity in the German Mobility Panel Tobias Kuhnimhof Institute for Transport Studies, University of Karlsruhe Paris, May 20th, 2005

2. Overview • The German Mobility Panel • MOP-recruitment, non-response and assumptions about selectivity • Additional sources of information to analyse selectivity in the MOP • Some possibilities to analyse selectivity issues • Some findings • Trustworthiness of CATI-Data • Who drops-out and who takes part • The advantages of recruiting households • Conclusions

3. 3 years  Survey of individual developments, transitions, causes and effects Year 2 Year 1 Year 3 The German Mobility Panel The MOP-survey: 7-day mobility diary in 3 consecutive years since 1994 7 days  Longitudinal survey of individual mobility behavior

4. Recruitment, Nonresponse and Assumptions about Selectivity The MOP recruitment: motivating relialable participants The multi-stage recruitment process: Commercial market research CATI recruitment (mailing of documents etc.) + 1-week-report + 3-year-survey = high participant burden MOP-recruitment CATI Written declaration of participation Participation: 1-Week-mobility-diary report, mailing of documents There is plenty possibility & it is understandable not to participate in the MOP

5. Recruitment, Nonresponse and Assumptions about Selectivity High respondent burden High drop-out Strong selective bias?

6. P Mobility in Population MOP-Recruitment, Non-response and Assumptions about Selectivity Common Assumptions about selective error in mobility surveys Stress due to job, education, children? Not interested in mobility issues High incomes ? Business trips? Not mobile due to incapacity (e.g. sick)? Large leisure activity spectrum?

7. Documentation of entire drop-out Additional sources of information to analyse selectivity in the MOP OBJECTIVE: Get to know the drop-outs BUT: preserve the survey’s continuity Documentation of calling attempts (max. 12) Mobility interview Enquiry of reasons for declining

8. Additional sources of information to analyse selectivity in the MOP Core-element of selectivity analysis: Mobility interview in commercial market research CATI  Data available about all CATI-interviewees (MOP-participants & drop-outs): - individual and household socio-economics - role of the person in the household - interviewee’s daily obligations (childcare, work etc.) - individual mobility on a test day (“yesterday”) - characterizing information about the person’s general mobility behavior - proxy-data on other household members

9. Logit 4 Logit 3 Logit 2 Logit 1 (Sampling Drop-out) Global drop out logit Some possibilities to analyse the selectivity issues Analysees to understand selective bias Modeling participation: P(participation)=f(mobility, socio-demografics)

10. CATI-Interviewees CATI and recruitment: All MOP-Participants (1.-,+ 2.-,+ 3.-time participants) Some possibilities to analyse the selectivity issues Respondent groups and data availability: Possibilities to compare mobility data of - different sources (survey methods) - and different samples First-time MOP-Participants (CATI-Interviewees + other household members) CATI-Interviewees, who participate in the MOP

11. Findings: Trustworthiness of CATI-data Are CATI-responses useful to characterize mobility behavior? Intrapersonal comparison of general mobility information (CATI) and reported mobility (MOP)

12. Findings: Trustworthiness of CATI-data Are CATI-responses useful to characterize mobility behavior? • - in most cases: YES • the stronger the questions relate to routines, the more reliable is the answer • active persons tend to overestimate themselves • less active persons tend to underestimate themselves • “Extrapolation of normal weekday” – Example: use of travel modes • Daily use of car = car use on 5,4 days per week • Daily use of PT = PT use on 4,5 days per week

13. Findings: Trustworthiness of CATI-data Differences in cross-sectional mobility figures in CATI and mobility diary data Comparision of test-day cross-sectional data of CATI vs. MOP

14. Findings: Trustworthiness of CATI-data Methodological Differences: The MOP-Report is more exact than the CATI Test day data of persons who participated in both surveys*: *No Fridays, Saturdays, exclusion of daily trips that exceed 6

15. Findings: Who drops out and who takes part What is the impact of the sample differences due to selectivity on mobility figures? Comparison of cross-sectional data CATI – MOP: After accounting for methodological differences the selective error can be estimated

16. Findings: Who drops out and who takes part Drop-Out of Non-trippers CATI test day data vs. MOP test day data of all persons in both surveys*: *No Fridays, Saturdays, exclusion of daily trips that exceed 6, appropriate weighting procedures applied in order to account for socio-demographic differences

17. Significant, P<0.1 Not significant P<0.2 P<0.3 Who drops out and who takes part Socio-economic aspects of Selectivity Example: Odds ratios of 2 out of 15 variables in participation logit model “Middle class bias”

18. Who drops out and who takes part Explanations for increasing share of mobile persons Example: Odds ratios of 2 out of 15 variables in participation logit model Significant, P<0.1 Not significant P<0.2 P<0.3 “Mobility interest bias”

19. CATI-Interviewees CATI and recruitment: The advantages of surveying households First-time MOP-Participants (CATI-Interviewees + other household members) CATI-Interviewees, who participate in the MOP  Participation of other household member can counter-balance selectivity impacts

20. Other Findings • CATI-data is useful but has to be interpreted • Surveying households counter-balances selectivity Recommendations • Balanced recruitment of different mobility styles is vital • Survey households not individuals • Don’t trade data quality for a high response rate Conclusions and Recommendations Findings on Selectivity • Socio-economic selective effects dominate drop-out • “Middle Class Bias” good education, good income, middle aged • “Mobility Interest Bias”  drop out of non-trippers (particularly elderly, permanent incapacity?)