“…A typical assumption in psychotherapy research is that change is gradual and linear, but individual time course data has revealed that the process of psychotherapy can also follow a nonlinear course [ 11 ]. Change in symptoms and therapy processes can show quadratic patterns (U- or V-shaped [ 76 ]), as well as cubic [ 77 , 78 ], saw-toothed [ 79 , 80 ], and other nonlinear patterns [ 81 – 83 ], all of which have been associated with better treatment outcomes. In addition, specific types of discontinuities or sudden jumps in time course data have been identified that predict symptom reduction across a variety of clinical disorders, including the sudden gain [ 74 , 84 – 86 ] (a large decrease in symptoms in a 1-week interval), the depression spike [ 78 ] (an intentionally induced and transient symptom exacerbation), and the cusp catastrophe pattern, which has been used to model sudden transitions from an abstinent state to relapse in addiction research [ 87 , 88 ].…”