The 'rate' measurement is useful when:

The 'rate' measurement is particularly useful when it comes to comparing frequency across varying time periods because it provides a standardized way of presenting how often a behavior occurs within a designated timeframe. By expressing behavior frequency as a rate—typically the number of occurrences per unit of time—practitioners can analyze trends and make more meaningful comparisons over different intervals.

For example, if a behavior occurs at a rate of 5 times per hour in one observation period and 10 times per hour in another, this information allows for an easy comparison and understanding of changes in behavior over time. This capability to effectively relate behavior frequency to the length of observation helps in identifying patterns and making informed decisions regarding interventions or supports.

The other options do not align with the primary function of rate measurement. Timing the duration of an event would be better suited for duration measures rather than frequency. Recording multiple behaviors during specific intervals leans more towards tallying occurrences rather than calculating rates. Lastly, assessing intensity involves different metrics that capture the strength or severity of a behavior, which is distinct from simply counting occurrences.