OBJECTIVES: Photoguides are used as measures of treatment effect. To interpret the results, an understanding of what change is “enough” is required. The purpose of this study was to determine the Patient Detectable Difference (PDD) and Patient Relevant Difference (PRD) in scar severity using a discrete choice experiment with paired comparisons of scar photos via the internet. METHODS: Patients were asked to select the scar most like their own from a 5-photo photoguide as the referent.   Each patient was then presented with a randomly selected scar from a scar library previously scored on severity by clinicians.  For the PDD, patients were asked if the random scar was better, worse, or about the same.  For the PRD, patients were asked if they would undergo treatment to “get” the random scar as opposed to keeping the referent scar. Each exercise was repeated five times. Optimal cutoff scores for PDD and PRD were based on a random intercept logistic model (RILM). RESULTS: Using a series of RILM estimations across 514 participants, the PDD was calculated as 15.4 points on a 100-point scale (area under curve = 0.85, sensitivity = 0.83, specificity = 0.73).  In subgroup analyses by  gender and scar source,, the PDD ranged from 9.5 to 22.0 points. The replicate analysis adjusting for directionality of preference (improvement only) showed a PRD of 26.9 points (area under curve = 0.97, sensitivity = 0.90, specificity = 0.93), which ranged from 19.5 to 32.0 in the subgroups. CONCLUSIONS: Results show that, as expected, scar patients report detectable differences in scar severity that are smaller than what they report as relevant differences necessary to choose treatment.  The Internet-based approach using a discrete choice experiment is a novel method to quantify the threshold of detectable and relevant differences in scar severity using a photoguide.