The proportion (usually expressed as a percentage) of externally requested items—documents, books, articles or other materials—that are successfully obtained from other institutions and delivered to the requesting patron within the service’s defined parameters (e.g., time window, edition, or format).

Principle

As an operational metric, the rate quantifies the effectiveness of external resource sharing networks and supply processes; it depends on both the availability of the requested item in partner collections and the agreed service-level definitions used to count a ‘successful’ transaction.

Demonstration

Illustrative scenario → Situation: A library records 200 interlibrary loan (ILL) requests in a month. Recognition: 150 of those are provided to patrons in the requested format and within the library’s two‑week service window. Action: The library computes Interloan Success Rate = 150/200 = 75%. Consequence: The library uses the 75% rate to evaluate partner performance and to identify needs for new agreements or alternative acquisition methods.

Misapplication

Interpreting the metric as a direct measure of patron satisfaction or collection adequacy. The semantic error is conflating ‘successful supply’ (a transactions outcome defined by the service) with quality, timeliness experienced by every patron, or the absence of unmet demand outside the measured denominator.

Consequence

Used to inform collection-development decisions, consortium agreements, staffing and budgeting for document delivery. A persistently low rate can cause increased direct acquisitions, changes in partner selection, or altered patron expectations; a high rate can mask long turnaround times or limited format availability if service definitions are narrow.

Reversal

When the denominator or success criteria change (e.g., counting only requested items that exist in partner catalogs, or extending the acceptable delivery time), the numeric rate can change independently of underlying network performance. The metric also does not apply unchanged to licensed digital resources, where supply is governed by licensing rather than physical availability.

Boundary

Clearly within: requests sent by a library to external institutions for physical or digital items where the response and delivery are tracked. Boundary case: requests routed through a consortium with different fulfillment rules—measurement depends on which actor’s logs are used. Clearly outside: internal loans within the same institution, automated licensed-access downloads counted as direct acquisitions rather than interloans.

Semantic Tension

Tension between maximizing success rate (completeness of supply) and controlling cost/turnaround (efficiency); improvements in one can require trade-offs in the other.

Synthesis

Interloan Success Rate is a service-definition–dependent operational indicator of how often external sourcing yields a counted supply; it is most useful when accompanied by clear definitions of ‘success’, the measurement window, and the composition of the request population.