Partial interval vs whole interval vs momentary time sampling
All three methods split an observation into equal intervals and score each one, instead of recording every occurrence. They differ only in what earns a score, and that is why they give different numbers for the same session.
| Method | Score the interval when | Tends to | Often used for |
|---|---|---|---|
| Partial interval | The behavior happens at any point in it | Overestimate | Behaviors to decrease |
| Whole interval | The behavior lasts the entire interval | Underestimate | Behaviors to increase, such as on-task |
| Momentary time sampling | The behavior is happening at the moment the interval ends | Either way | Groups, or when you can only glance up |
One session, three answers
A 6-minute observation uses 1-minute intervals. A client is out of their seat from 0:40 to 2:10 and from 4:30 to 4:50. That is 110 seconds out of 360, or about 31% of the session.
| Interval | Partial | Whole | Momentary (at the end) |
|---|---|---|---|
| 0:00 to 1:00 | Yes | No | Yes |
| 1:00 to 2:00 | Yes | Yes | Yes |
| 2:00 to 3:00 | Yes | No | No |
| 3:00 to 4:00 | No | No | No |
| 4:00 to 5:00 | Yes | No | No |
| 5:00 to 6:00 | No | No | No |
| Result | 4 of 6, 67% | 1 of 6, 17% | 2 of 6, 33% |
Partial interval reads well above the real 31% and whole interval well below it. Momentary time sampling lands close here, but it missed the 20-second episode completely, so it can miss short behaviors.
How exam-style questions ask this
- If the question says "at any point", it is partial interval.
- If it says "the entire interval", it is whole interval.
- If a timer beeps and you record only what is happening right then, it is momentary time sampling.
- To report the result, divide scored intervals by total intervals: on task in 18 of 24 intervals is 75% of intervals.
Try the free 20-question practice test, or read about DRA, DRI, DRO and DRL and the four functions of behavior.
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