This weeks Aviation Week informs us that the Pk, the probability of kill, of the Iron Dome is .78. Out of 30 incoming rounds fired at, 6 got through. To achieve that success rate it was necessary to fire two interceptors at each incoming round. The third of the planned 13-15 Iron Dome systems should be delivered by Raphael in October. The success rate last spring had been 88%. With few exceptions operators and not equipment was blamed.
from the article: Israel deploys third Iron Dome anti-rocket battery near Ashdod