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This paper investigates the impact of dynamic swarm size changes on human operator workload in human-swarm interaction. Through two user studies (N=34) involving a simulated drone monitoring task, the researchers manipulated swarm size between episodes and measured perceived workload. Results indicate that small swarm size increases preserve lower workload, while small decreases elevate workload, and large changes in either direction induce a cognitive reset, attenuating these effects.
Small decreases in swarm size leave human operators with elevated workload, even when performance is unaffected, suggesting a "workload residue" effect that designers must address.
Real-world deployments of human--swarm teams depend on balancing operator workload to leverage human strengths without inducing overload. A key challenge is that swarm size is often dynamic: robots may join or leave the mission due to failures or redeployment, causing abrupt workload fluctuations. Understanding how such changes affect human workload and performance is critical for robust human--swarm interaction design. This paper investigates how the magnitude and direction of changes in swarm size influence operator workload. Drawing on the concept of workload history, we test three hypotheses: (1) workload remains elevated following decreases in swarm size, (2) small increases are more manageable than large jumps, and (3) sufficiently large changes override these effects by inducing a cognitive reset. We conducted two studies (N = 34) using a monitoring task with simulated drone swarms of varying sizes. By varying the swarm size between episodes, we measured perceived workload relative to swarm size changes. Results show that objective performance is largely unaffected by small changes in swarm size, while subjective workload is sensitive to both change direction and magnitude. Small increases preserve lower workload, whereas small decreases leave workload elevated, indicating workload residue; large changes in either direction attenuate these effects, suggesting a reset response. These findings offer actionable guidance for managing swarm-size transitions to support operator workload in dynamic human--swarm systems.
