Objective To measure the validity of a booster seat questionnaire and a car seat questionnaire so that they can be reliably used in future trials.

Design The two child passenger safety questionnaires were created. Each underwent expert review to ensure face and content validity. Two reliability studies were conducted independently of each other. Care givers for children who were cared for in a paediatric emergency department were enrolled. Criterion validity was tested by direct observation, and inter-rater reliability was measured. Test–retest reliability and internal consistency were also measured on the booster seat questionnaire.

Results Booster seat questionnaire: test–retest agreement was good, with 16 of the 18 questions having agreement of at least 80%. Agreement was acceptable (>70%) in all other questions in which agreement was expected when the answers of two different care givers were compared. All care givers were able to report booster seat use as it was found on direct observation, yielding a κ value of 1. Car seat questionnaire: the respondents showed 95% agreement between their reported type of child safety restraint and what was found on direct observation. Inter-rater agreement for the type of safety restraint was 77% or a κ value of 0.7.

Conclusion Both the booster seat questionnaire and the car seat questionnaire are valid and can be reliably used in future studies. The findings of this study need to be verified with larger studies and different populations.