Discontinuous Measurement in ABA: Partial Interval, Whole Interval and Momentary Time Sampling
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What is discontinuous measurement in ABA?
Discontinuous measurement records a sample of behavior instead of every instance. The observation is split into equal intervals, and each interval is scored as yes or no. Partial interval recording scores any occurrence, whole interval recording scores only behavior that lasts the whole interval, and momentary time sampling scores behavior at the moment the interval ends.
Continuous measurement, such as frequency or duration recording, captures every instance of a behavior. That is often impractical when a technician is also teaching, or when a behavior is very frequent or has no clear start and end. Discontinuous methods trade some accuracy for a lighter observing load. The BACB RBT Test Content Outline (3rd ed.), which sets the RBT exam content from 2026, lists "Implement discontinuous measurement procedures (e.g., partial & whole interval, momentary time sampling)" as task A.2.
The result is always a percentage of intervals, not a count or a number of minutes. Writing "aggression occurred 30% of the time" from partial interval data is a common error: the correct statement is "aggression was scored in 30% of intervals".
The three discontinuous measurement methods
| Method | Score the interval when | Error tendency | Usually used for |
|---|---|---|---|
| Partial interval | The behavior occurs at any point in the interval | Overestimates duration | Behaviors to decrease, brief or frequent behaviors |
| Whole interval | The behavior lasts the entire interval | Underestimates duration | Behaviors to increase, such as on-task or engagement |
| Momentary time sampling | The behavior is happening at the moment the interval ends | Over and under about equally | Ongoing behaviors, observing several learners |
How does each method score the same observation?
The table follows one fictional 6-minute observation of out-of-seat behavior, split into 1-minute intervals. Measured continuously, the learner was out of seat for 153 of 360 seconds, or 42.5% of the time.
| Interval | What happened | Partial | Whole | MTS |
|---|---|---|---|---|
| 1 | Out of seat for 10 seconds mid-interval | Yes | No | No |
| 2 | Out of seat the whole minute | Yes | Yes | Yes |
| 3 | Seated throughout | No | No | No |
| 4 | Left seat at 0:40 and was still up at 1:00 | Yes | No | Yes |
| 5 | Out of seat the whole minute | Yes | Yes | Yes |
| 6 | Out of seat for 3 seconds at 0:05 | Yes | No | No |
| Result | Continuous duration: 42.5% | 83% of intervals | 33% of intervals | 50% of intervals |
Which way does each method err, and why it matters
Powell, Martindale and Kulp (1975) compared time-sample measures with a continuous measure. Whole-interval sampling consistently underestimated the behavior, partial-interval sampling consistently overestimated it, and momentary time sampling over- and underestimated about equally often. Sampling more often brought every method closer to the continuous measure.
That pattern explains the usual pairing: partial interval for behaviors you want to reduce (an overestimate is the conservative error), and whole interval for behaviors you want to increase (an underestimate is conservative). Meany-Daboul and colleagues (2007) found that treatment decisions drawn from momentary time sampling and partial interval data often matched those drawn from continuous data; MTS was slightly more likely to match duration data and partial interval slightly more likely to match frequency data.
How do you choose an interval length?
Shorter intervals give more accurate data but demand more attention. Many programs use intervals between 10 seconds and a few minutes depending on the behavior and how many tasks the observer is doing. Whatever length is chosen, keep it constant across sessions so the percentages can be compared on one graph, and use a timer or vibrating cue rather than estimating.
If a behavior is rare and has a clear start and end, frequency recording on an ABC data sheet is often more informative than any interval method.
Is discontinuous measurement the same as time sampling?
Interval recording and time sampling are related but named differently across textbooks. In common ABA usage, partial and whole interval recording score the whole interval, and momentary time sampling scores a single moment. A planned activity check (PLACHECK) is a form of momentary time sampling used to score how many members of a group are engaged at the end of each interval.
How do you write it in an ABA session note?
- Step 1: Name the target behavior and its operational definition, or refer to the definition in the plan.
- Step 2: State the method, interval length and observation length (for example, 10-second partial interval over 10 minutes).
- Step 3: Report the result as a percentage of intervals, never as minutes or a count.
- Step 4: Note anything that affected the sample: interruptions, missed intervals, a change of setting.
- Step 5: Compare with the previous sessions only if the method and interval length were the same.
Example session note (fictional, for illustration only)
Fictional 97153 session, clinic, learner age 8, plan targets reduction of vocal stereotypy and an increase in on-task behavior.
Vocal stereotypy measured by 10-second partial interval recording during two 10-minute table-work blocks (120 intervals planned). A fire drill interrupted 1 minute of the second block, so 6 intervals were not observed and are excluded: stereotypy scored in 34 of 114 observed intervals (30%), down from 41% last session. On-task behavior measured by 1-minute momentary time sampling during the same blocks: the check that fell during the fire drill is excluded, and the learner was on task at 15 of 19 checks (79%). Graph updated; data shared with supervising BCBA.
Common mistakes
- Reporting interval data as a duration ("out of seat 28% of the time") or as a frequency.
- Changing the interval length between sessions and graphing the results together.
- Using partial interval recording for a behavior you want to increase, which can make progress look better than it is.
- Estimating interval boundaries instead of using a timer.
- Leaving missed or interrupted intervals in the denominator without saying so.
- Using interval recording for a rare, discrete behavior where a count would say more.
Frequently Asked Questions
What is the difference between partial and whole interval recording?
Partial interval recording scores an interval if the behavior happens at any point in it, so it tends to overestimate. Whole interval recording scores an interval only if the behavior lasts the entire interval, so it tends to underestimate. Both report the percentage of intervals scored.
Does momentary time sampling overestimate or underestimate?
Neither consistently. Powell, Martindale and Kulp (1975) found that momentary time sampling overestimated and underestimated about equally often, and that shorter intervals made every time-sample method more accurate.
What is continuous measurement in ABA?
Continuous measurement records every instance of a behavior during the observation, for example frequency, rate, duration and latency. Discontinuous measurement records only a sample, scored by interval. Use continuous measurement when it is practical and the behavior has a clear start and end.
How do you calculate the percentage for interval recording?
Divide the number of intervals scored by the number of intervals observed, then multiply by 100. Exclude intervals that could not be observed from both numbers, and say so in the session note.
Can WellNotes collect interval data?
No. WellNotes does not collect ABA data. After you collect data on paper or in your data system, you can enter the session details into the free ABA session note generator to draft the session note, then review and edit it.
Related ABA references and tools
Sources
- Cooper JO, Heron TE, Heward WL. Applied Behavior Analysis (3rd ed.). Pearson (Open Library record)
- Behavior Analyst Certification Board. RBT Test Content Outline (3rd ed.), RBT examination content from 2026
- Powell J, Martindale A, Kulp S. An evaluation of time-sample measures of behavior. J Appl Behav Anal 1975;8(4):463-469
- Meany-Daboul MG, Roscoe EM, Bourret JC, Ahearn WH. A comparison of momentary time sampling and partial-interval recording for evaluating functional relations. J Appl Behav Anal 2007;40(3):501-514
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