You are reading a single comment by @Clockwise and its replies. Click here to read the full conversation.
  • Looks like selection bias. If you read the way people had been recruited to take part it appears all had an interest in obtaining a free high vis jacket and so it's likely that they would disproportionately see the jackets as useful and ride/report/influence in a way that would give skewed data.

    In total, 11,202 cyclists signed up for the study. Only cyclists who would ride their bicycle at least three times a week in the summer and who were at least 18 years old when signing up were recruited. 366 registrations were rejected because they failed to meet the criteria, see Fig. 2. The cyclists were stratified on the jacket size (S-XXXL) since the jackets were produced in advance and therefore limited in amount. Thus, not all cyclists who signed up were selected to participate. 8042 participants were randomly assigned to the test and control groups. Of these, 6793 participants confirmed their participation (test group, n = 3402, control group, n = 3391). The test group participants received the yellow bicycle jacket to wear during the study period, while the control group used their regular bicycle garments with the prospect of receiving a yellow bicycle jacket after the completion of the study.

    And then.

    When signing up for the study, participants completed a web-based questionnaire containing demographic information: subject characteristics (gender, age), car ownership (yes/no), bicycle riding frequency (intervals) and jacket size needed.

    To further skew the data collected one group receives the reward for joining and one at the end for participating during the reporting period. It's in the interest of those without a high vis jacket to continue participation and interaction via reporting incidents as that would result in a high vis jacket at the end, for the group who has been given them at the start they have a different lower level of incentive to continue participation.

    Only issue I can see is that the voices who shout loudly for more high vis will champion it's results and it's so wordy most won't look beyond the sensational headline of it.


Avatar for Clockwise @Clockwise started