Developing and validating self-report instruments assessing perceived driver competence

Abstract: The overall aim of this thesis was to develop and validate a self-report instrument for perceived driver competence. The thesis includes six papers and a summary. All papers focus on perceived driver competence from a measurement perspective; that is, how to develop an instrument for perceived driver competence and how to use and interpret the scores from the instrument in a reliable and valid manner.Study I reviews how perceived driver competence has been measured in other studies and discusses these methods from a measurement perspective. Most studies have examined perceived driver competence by asking drivers to compare their own skill to that of the average driver. That method is problematic, since it is not possible to determine if drivers are overconfident or not, when empirical information of their own skills is missing. In order to examine if drivers overestimate their skills or not, perceived driver competence should be compared with actual driving performance.Study II reports on the development and psychometric evaluation of a self-report instrument for perceived driver competence - the Self-Efficacy Scale for Driver Competence (SSDC). The findings provides support for construct validity, as the SSDC demonstrated sound psychometric properties and as the internal structure of the SSDC corresponded to the theoretical model used as a basis for instrument development.In study III, the psychometric properties of the SSDC were further examined using an item response theory (IRT) model. The findings confirmed the results indicated by the classical analyses in Study II. Additional information was provided by the IRT analyses, as it was indicated that the scale would benefit from fewer scale points or by putting labels on each scale point.In study IV, Swedish and Finnish candidates’ self-assessment accuracy was examined by comparing candidates’ scores on the SSDC and a similar instrument for self-assessment of driving skill used in Finland, with driving test performance. Unlike previous studies, in which drivers compared their perceived skills to that of the average driver, a relatively large proportion made a realistic assessment of their own skills. In addition, in contrast to previous studies, no gender differences were found. These results were also confirmed in study V, where the results from the Finnish instrument for self-assessment of driving skill were compared with the results from a similar instrument used in the Netherlands.Study VI further examined the construct validity of a revised version of the SSDC, combining qualitative and quantitative sources of evidence. There was a strong relationship between the SSDC and an instrument for self-assessment of driving skills, providing support for convergent validity. No relationship was found between the SSDC and driving test performance. Explanations of the lack of relationship were provided from semi-structured interviews, as they indicated that confidence in performing different tasks in the test are different from being confident of passing the test, and that the candidates are familiar neither with assessing their own skills nor with the requirements for passing the test.In conclusion, the results from this thesis indicated that the choice of methods for assessing perceived driver competence as well as the quality of these methods affect the validity. The results provided support for different aspects of construct validity of the SSDC. Moreover, the findings illustrated the benefits of combining different methods in test validation, as each method contributed information about the validity of the SSDC. The studies in this thesis mainly examined internal and external aspects of construct validity. Future studies should examine procedural validity of the SSDC.