Table 1

 Two dimensional hierarchy of study utility

Study designStudy population
*The effectiveness of interventions given in a clinical setting was outside the scope of the review.
Randomised controlled trialHouseholds or local residents
Controlled panel study (repeated measures on the same participants)Subset of local population (drivers, commuters, or school pupils)
Controlled repeated cross sectional study Controlled retrospective study Uncontrolled panel study (repeated measures on the same participants) Uncontrolled repeated cross sectional studyParticipants in a targeted intervention already selected from one of the groups above Passers by at, or visitors to, a study location Patients* Vehicles
Uncontrolled retrospective studyPopulation not clear
Case study of trends in mode share
Design not clear