Phase change line
A phase change line is a vertical line on a graph that marks the point where one condition ends and another begins, such as baseline to intervention.
A phase (or condition) is a period in which the same procedures are in effect, such as baseline, intervention, or a change in the intervention. On a line graph, a vertical line, often dashed, is drawn between the last session of one phase and the first session of the next. Each phase is labeled at the top of the graph so the reader knows what was in effect.
Data points on either side of a phase change line are not connected by a data path. Joining them would suggest that the behavior moved continuously from one condition to the other, when in fact the conditions were different. This lets the reader compare level, trend and variability before and after the change.
Phase change lines are added when the supervisor changes the plan: starting an intervention, thinning a reinforcement schedule, adding a new procedure or returning to baseline. When an RBT updates a graph, they draw or add the line only for changes the supervisor made and label it as instructed. Other important events that were not a planned change, such as a new medication or an illness, are usually noted with a brief annotation rather than a phase change line, and are reported to the supervisor.
Example and non-example
| Example | An RBT updating a graph adds a dashed vertical line between session 8 and session 9 and labels the new phase "FCT" because the supervisor started functional communication training at session 9. |
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| Non-example | Drawing a line straight across the graph at the level of 80% to show the mastery goal is an aim line or criterion line, not a phase change line. |
Don't confuse it with
Test yourself
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Question 1What does a phase change line on a line graph show?
Show answer
C: A change in conditions, such as starting intervention. A phase change line is a vertical line drawn between the last session of one condition and the first session of the next, showing exactly when conditions changed.
- A: Meeting a criterion is not a change in conditions and is not what a phase change line marks.
- B: A phase change line marks a change in conditions, not the peak of the data.
- D: A goal level would be a horizontal criterion line, while a phase change line is vertical.
Task A.4 · 40-hour curriculum section C
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Question 2The supervisor has started intervention, and an RBT is plotting the first intervention session on a graph that shows baseline data. How should this new data point relate to the last baseline point?
Show answer
D: It is not connected; a phase change line separates them. Data points are connected only within the same condition. The data path stops at the phase change line and a new data path begins in the intervention phase.
- A: Connecting across a phase change line would blend two conditions into one data path.
- B: No line of any kind joins data paths across a phase change; the only line there is the vertical phase change line.
- C: Baseline data stay on the graph; the new session is added as its own point.
Task A.4 · 40-hour curriculum section C
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Question 3How many sessions of baseline data are shown on this graph?
Number of hits by Session: 1: 6, 2: 7, 3: 5, 4: 6, 5: 3, 6: 2, 7: 2, 8: 1. Phases: Baseline from session 1, Intervention from session 5. Show answer
C: 4. Baseline runs from the first session up to the phase change line, which falls after the fourth session.
- A: This undercounts; four data points appear before the phase change line.
- B: The fifth point is the first point of the intervention phase.
- D: Eight is the total number of sessions across both phases.
Task A.4 · 40-hour curriculum section C
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On the RBT exam
This term belongs to task A.4 Entering data and updating graphs in content area A, Data Collection and Graphing, which is 13 of the 75 scored questions.