Fig. 1
From: Clinical measurements obtained from point-of-care ultrasound images to assess acquisition skills
Theoretical measurement model and corresponding specification of multilevel statistical models. Rectangles represent groupings (or ‘clusters’), the ovals represent vein diameter measurements (the most granular unit of analysis), and connecting lines represent nested relationships. In the mixed-effect models, groupings were defined by both random effects and, at level 2 only, fixed effects. There were two fixed effects at level 2, one for each trainee that represented the adjusted differences between diameter means obtained from instructor-acquired ultrasound images. Because instructor-acquired ultrasound images served as the reference standard, the models assume that the instructor made no systematic errors of acquisition. (See text for additional assumptions made for this model.)