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GlossaryResponse performance and resources

Concurrent calls

Concurrent calls overlap in time. Learn how they affect emergency-resource availability and how to interpret overlap statistics.

Concurrent calls are separate calls or incidents whose active periods overlap. In emergency-service planning, the term describes demand occurring at the same time rather than one assignment finishing before the next begins. It is also called simultaneous calls or overlapping calls.

The counted period matters. A service may measure overlap while incidents are open, while units are assigned, or while particular ambulances are unavailable. American Fork's fire-station information uses concurrent calls to describe more than one call at a time. That plain-language definition still needs timing rules before it can become a reproducible statistic. American Fork fire-station information

Why overlap matters

An annual call total describes volume. It does not show when the calls arrive or how long they occupy resources. Two communities with the same yearly total can experience different pressure if one has demand spread evenly and the other has clusters of calls during a few busy hours.

Overlap becomes operationally important when the required resources are already committed. An incident needing one available engine presents a different problem from an incident needing an ambulance when all ambulances are transporting patients. Counting calls without identifying the resources needed cannot fully describe that difference.

Concurrent calls do not necessarily mean a service ran out of capacity. A department can have several staffed units available at once. Likewise, a single complex incident can use most available resources even though it counts as only one call.

Example

Hypothetical: An ambulance is assigned to a medical call at 09:00 and becomes available at 09:50. A second medical call is assigned at 09:20 and its ambulance becomes available at 09:45. The assignments overlap for twenty-five minutes, from 09:20 to 09:45.

There are two distinct calls and one continuous period with two calls active. A report counting calls that overlap another call could count both calls. A report counting minutes with two active assignments would report twenty-five minutes. Neither statistic should be described as the other.

If a third ambulance remains available throughout, this overlap does not establish a shortage. If no suitable ambulance is available when another call arrives, the resulting wait or aid request is a separate operational outcome worth measuring.

Common confusion

Multiple callers reporting one event are not necessarily concurrent incidents. Several people may call about the same crash, and dispatch staff may link their reports into one incident. San Francisco's calls-for-service documentation explains that incoming calls and recorded dispatch incidents are different units of information. San Francisco dataset explainer, revised September 2021

Multiple units at one incident are also not multiple concurrent calls. A building fire with six assigned vehicles is one incident with several resources committed. A useful workload analysis may need both incident counts and unit assignments.

The proportion of calls with overlap differs from the proportion of time the system has overlapping calls. Long incidents and short incidents affect those measures differently. A percentage without a denominator leaves that distinction unresolved.

Reading an overlap analysis

Look for the start and end events, included call types, and geographical coverage. Check whether an assignment remains active during transport or hospital turnaround, and how canceled calls are treated. Those choices can materially change the result.

Useful summaries show the number of simultaneous assignments over time, resources still available, and any associated delays. Compare equivalent time periods and service boundaries. A rise in recorded overlap after adding a neighboring district may reflect the expanded reporting area, rather than a change in demand within the original community.

Sources