7.3 Discontinuous Measurement Systems (Interval Recording and Time Sampling)

7.3 Discontinuous Measurement Systems (Interval Recording and Time Sampling)

In Applied Behavior Analysis (ABA), while continuous measurement records every instance of behavior, clinical constraints often make continuous tracking impractical. When target behaviors occur at extremely high rates, lack discrete start and stop points, or when an Applied Behavior Analysis Technician (ABAT) is managing multiple instructional responsibilities, discontinuous measurement is utilized. Discontinuous measurement procedures sample behavior during specified intervals of time rather than recording every occurrence. ABATs must thoroughly understand the procedures, estimation biases, and clinical applications of the three primary discontinuous measurement systems: Partial-Interval Recording (PIR), Whole-Interval Recording (WIR), and Momentary Time Sampling (MTS) (including group PLACHECK).

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1. Fundamentals of Discontinuous Measurement

In discontinuous measurement, an observation session (e.g., 30 minutes) is divided into smaller, equal time intervals (e.g., 10-second, 30-second, or 1-minute intervals). The ABAT observes whether the target behavior occurs according to specific interval rules and records a binary score (plus + for occurrence, minus - for non-occurrence) for each interval.

Calculation Formula for Interval Data

Data from discontinuous measurement are expressed as a percentage of intervals:

$\text{Percent of Intervals Scored} = \left( \frac{\text{Number of Intervals Scored (+)}}{\text{Total Number of Intervals Observed}} \right) \times 100$

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2. Partial-Interval Recording (PIR)

Procedural Rules

In Partial-Interval Recording (PIR), the ABAT scores an interval as positive (+) if the target behavior occurs at any point during the interval, regardless of how brief the duration or how many times it occurs. If the behavior occurs for 1 second during a 10-second interval, the entire interval is scored as positive.

Systematic Estimation Bias

• PIR systematically OVERESTIMATES overall duration: Because a behavior lasting 1 second scores the same as a behavior lasting the full 10 seconds, PIR overestimates total time spent in behavior.
• PIR systematically UNDERESTIMATES high-frequency behavior: Multiple occurrences within a single interval are recorded as only one positive interval.

Clinical Application

PIR is primarily used for behavior reduction targets (e.g., vocal outbursts, hand-flapping, self-injurious behavior). Because PIR overestimates behavior, if PIR data show a decreasing trend, the clinical team can be highly confident that the problem behavior is truly decreasing.

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3. Whole-Interval Recording (WIR)

Procedural Rules

In Whole-Interval Recording (WIR), the ABAT scores an interval as positive (+) ONLY if the target behavior occurs continuously throughout the entire duration of the interval from start to finish. If the behavior stops for even 1 second during a 10-second interval, the interval is scored as negative (-).

Systematic Estimation Bias

• WIR systematically UNDERESTIMATES total duration and occurrence: Brief interruptions cause intervals to be scored as negative, even if the behavior was present for 90% of the interval.

Clinical Application

WIR is primarily used for skill acquisition and behavior expansion targets where continuous persistence is desired (e.g., on-task study behavior, cooperative play, sustained eye contact, independent work engagement). Because WIR underestimates behavior, if WIR data show an increasing trend, the team can be confident the client is truly expanding their behavioral endurance.

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4. Momentary Time Sampling (MTS)

Procedural Rules

In Momentary Time Sampling (MTS), the ABAT sets an interval timer (e.g., every 1 minute or 5 minutes). The ABAT does not monitor the client during the interval. When the timer chimes at the exact moment marking the end of the interval, the ABAT looks up and scores positive (+) if the behavior is occurring at that precise split second, and negative (-) if it is not.

Systematic Estimation Bias

• MTS does not systematically overestimate or underestimate duration in a consistent direction, provided intervals are brief. However, it can miss low-frequency, short-duration behaviors that occur between timer chimes.

Clinical Application

MTS is ideal when ABATs have high instructional loads (e.g., running Discrete Trial Training or managing classroom groups) and cannot continuously watch a student. It is widely used for classroom on-task behavior and peer interaction.

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5. Planned Activity Check (PLACHECK)

Procedural Rules

PLACHECK is a variation of Momentary Time Sampling designed for group settings (e.g., group therapy sessions, special education classrooms). At the end of a predefined interval, the ABAT counts the number of individuals in a group who are actively engaged in the target behavior and divides it by the total number of individuals present.

$\text{PLACHECK Percentage} = \left( \frac{\text{Number of Group Members Engaged at Moment}}{\text{Total Number of Group Members Present}} \right) \times 100$

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6. Discontinuous Measurement Systems Comparison Table

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7. Clinical Scenario Demonstrating Interval Scoring Differences

Consider a 10-minute observation divided into 1-minute intervals (10 intervals total) measuring on-task behavior:

• Interval 1: Client on-task for seconds 5 to 15 (10 seconds total).
• Interval 2: Client on-task for seconds 0 to 60 (full 60 seconds).
• Interval 3: Client on-task for seconds 10 to 55 (45 seconds total).
• Interval 4: Client off-task entire minute (0 seconds).

Scoring comparison:

• PIR: Scored + in Intervals 1, 2, and 3 ($3 / 4 = 75%$).
• WIR: Scored + ONLY in Interval 2 ($1 / 4 = 25%$).
• MTS (timer chimes at 60s): Scored + ONLY in Interval 2 ($1 / 4 = 25%$).

This illustrates how the choice of measurement system profoundly alters reported behavioral percentages. ABATs must strictly adhere to the exact measurement procedure prescribed in the client's Behavior Intervention Plan (BIP).