1942: British troops in Southeast Asia needed to guard their positions against Japanese night attacks. An obvious defence was to rig up perimeter wires that would light signal lamps when breached by enemy soldiers. Less obvious was the correct thickness of the wires. Too thin and they would break accidentally; too thick and they would be spotted by the enemy.
In Britain, operational research scientists experimented by stringing wires across routes used by the charladies who scrubbed the floors of government establishments. The cleaners spent much of their time moving forward on their knees – rather like the Japanese creeping through the jungle undergrowth. By observing which wires the cleaners noticed and which ones they unwittingly broke, the scientists were able to determine the correct thickness for the wires.
Source: J.G. Crowther and R. Whiddington, Science at War (1947), p. 112