Capturing Competency Mapping for Question Response Reports

Hi I have a requirement I'm not sure how we could achieve. We have an online assessments application for which we track all the submitted question responses by users, including their result. We use this to report on pass rate by question and also roll this up to the assessment. We want to expand this to report not just by the question or assessment but also by the competencies assessed. This would be pretty straightforward if each question could only map to one competency but unfortunately, a question could be mapped to 0, 1 or many competencies What is the best way to capture these competencies against each submitted question response so that we can report not only on question pass rate but also on competency pass rate?