What do solid vertical lines on a graph represent?

The correct answer is that solid vertical lines on a graph represent phase changes. In the context of applied behavior analysis and other scientific fields, phase changes are used to denote significant transitions in conditions or interventions. These transitions may indicate shifts from one phase of a study to another, such as moving from a baseline (no intervention) to an intervention phase. The use of solid vertical lines helps to clearly demarcate these changes on the graph, allowing viewers to easily identify at what points different conditions were applied, which is crucial for analyzing the effects of interventions over time.

Phase changes are important for visual representation in data analysis because they also allow for better interpretation of results, as one can evaluate how behavior changes following the introduction or withdrawal of a treatment. This facilitates a clear understanding of the behavior being measured in relation to the varying conditions imposed in a structured setting.