We describe an application of the assessment triangle, using a learning progression as the "cognition" vertex. We summarize two studies to evaluate whether evidence of student performance is consistent with our progression. In Study 1, we conducted think alouds using draft assessment activities and evaluated responses in relation to the sequencing of several key progression stages. In Study 2, we piloted revised activities and conducted latent class analysis to classify students into categories based on stage mastery. Results were mixed, with evidence generally suggesting that the progression is more of a loose network of concepts than a strict hierarchy.