Critical Separation Versus Effective Sweep Width: Bridging the Old and New Search-and-Rescue Worlds

"Critical separation" is a simple method for spacing searchers intended to produce a predictable probability of detection (POD). A more precise method, based on effective sweep width (W), has been adopted only slowly. Accurate PODs are critical in planning priorities in future operational periods. Therefore, it would be useful to know what POD a critical separation determination actually produces.

Detection ranges (Rd) were measured on high- and low-visibility adult mannequins (stuffed Tyvek suits of varying colors) and a red helmet detected by 6 searcher volunteers. Conversion factors previously described in the literature were used to convert Rds into predicted Ws and the previously described logit model to convert Ws into PODs.

The Rds were 26±6 m (mean±SD) for the high-visibility mannequin, 15±5 m for the low-visibility mannequin, and 24±6 m for the helmet used in the critical separation determination. The predicted Ws were 47±15 m, 16±8 m, and 42±14 m, respectively. Critical separation would therefore result in predicted PODs of 67±17% or 32±16% for nonresponsive adults in high- versus low-visibility clothing.

Critical separation is a simple, quick way to array searchers in the field. The current study suggests that the value of critical separation in producing the 50% target POD will depend on the whether the search object used for the critical separation determination has a similar detectability/W to the actual target of the search effort. Still, the results from a critical separation determination may offer more predictability and reliability than the common practice of POD estimation by team leaders.