Discontinuous Measurement Traps: Whole-Interval, Partial-Interval, and Momentary Time Sampling

The Underestimation / Overestimation Trap

Questions on discontinuous measurement are virtually guaranteed on both the BCBA and RBT exams. Test writers love them because they have clear mathematical rules regarding measurement bias.

The Three Methods and Their Inherent Artifacts

1. Whole-Interval Recording

• Scored as (+) ONLY IF: The behavior occurs throughout the ENTIRE duration of the interval.
• Measurement Bias: UNDERESTIMATES the overall percentage and duration of the behavior. If an interval is 10 seconds and the student works on-task for 9.8 seconds, the interval is scored as 0 (minus).
• Clinical Recommendation: Use when you want to increase a continuous behavior (e.g., on-task engagement, reading fluency, sustained peer play). If whole-interval data shows progress, you are 100% certain the client is improving because the bar is set so high.

2. Partial-Interval Recording

• Scored as (+) IF: The behavior occurs at ANY point during the interval, even for a fraction of a second.
• Measurement Bias: OVERESTIMATES the overall percentage of the behavior. If a child engages in aggression for 0.5 seconds in a 60-second interval, the entire interval is scored (+).
• Clinical Recommendation: Use when you want to decrease a high-priority problem behavior (e.g., self-injurious behavior, vocal outbursts). If partial-interval shows a drop to zero, you are certain the behavior is truly extinguished.

3. Momentary Time Sampling (MTS)

• Scored as (+) ONLY IF: The behavior is occurring at the EXACT instant the interval ends.
• Measurement Bias: Can either overestimate or underestimate, but does not require continuous observation.
• Clinical Recommendation: Ideal for teachers or single practitioners managing multiple students simultaneously.

High-Yield Memory Anchor

• Whole Interval \rightarrow High bar \rightarrow Underestimates \rightarrow Use for behaviors to INCREASE.
• Partial Interval \rightarrow Low bar \rightarrow Overestimates \rightarrow Use for behaviors to DECREASE.