The relationship between a resident physician and his/her supervising attending is foundational to graduate medical education and may impact the clinical learning environment and resident well-being. This paper focuses on how to measure connection between a resident and their clinical supervisor. Connection includes the subdomains of psychological safety, empathy, educational alliance, and feedback. After reviewing the literature, the authors designed the 12-item, 7-point Connection Index (CI12) to quantitatively measure connections between a resident and his/her supervisor during a 6-month period (supervision dyad), and based on educational alliance, empathy, psychological safety, and effective feedback. A 9-criteria evaluation framework was applied to assess its reliability and validity on a sample of psychiatry residents at a residency program, July 2016 through June 2018. Out of a total possible number of 50 residents, 100% participated to rate 41 supervisors over 201 supervision dyads; the CI12 satisfied all eight of the eight testable criteria, including high scalability (H = 0.78), consistency (alpha = 0.98), test-retest validity (ICC = 0.95), and construct validity where CI12 was found to have statistically significant correlations with outcomes measures (greater connection was associated with less negative emotional experiences, less mistreatment or bias, less burnout, and higher attendance to supervision sessions). The authors showed the CI12 can be a valid and reliable instrument to quantify whether a resident and his/her supervisor connects during a 6-month supervision with respect to empathy, psychological safety, educational alliance, and feedback. We recommend assessing connections as part of the overall evaluation of a resident's experience with the clinical learning environment.