About an hour and half after the cease fire at 9:00 pm on November 21 2012, the Israeli Defense Forces (IDF) released an announcement with some details on the rocket attacks from the Gaza Strip and on the effectiveness of the Iron Dome system in countering them.[1]  Here are figures from that announcement, which start with the beginning of the attacks on November 14 and goes up to the time of the cease fire:

In the same announcement, the IDF stated that : The ‘Iron Dome’ defense system has accomplished high rate of successful interceptions (84%) and Hamas’ accuracy with regard to hitting populated areas within Israel remains below 7%.” One of the injured persons, an Israeli soldier injured shortly before the cease fire, subsequently died, bringing the total number of fatalities to six. 

It is unclear (at least to me) exactly what the 421 “interceptions” in the announcements means.  Since the numbers of rockets add up correctly (875+58+421+152 = 1,506), it seem clear that the number 421 refers to the number of rockets, not the number of Iron Dome interceptors.   If we interpret “intercept” to mean a successful intercept (unlike the way the terminology is sometimes used by the U.S. military), and assume the system attempted to intercept all the rockets that fell in urban areas and that were no intercept attempts against the rockets that fell in open areas, then we get a success rate of 421/(421+58) = 87.9%, higher than the claimed success rate of 84%.

If alternatively, “intercept” actually means an “intercept attempt” which is not necessarily successful, then we get a success rate of (421-58)/421 = 86.2%, still higher than the claimed success rate.

One possible explanation for these discrepancies is that intercept attempts may have been made on some of the rockets that landed in open areas.  (Given the likely somewhat erratic nature of the rockets’ trajectories and the limits of radar tracking accuracy, some of the rockets that were predicted to fall towards the outer edge of the defended area would inevitably fall outside of it.  In addition, to avoid unengaged rockets striking urban areas, the defended areas may have been extended beyond the actual urban areas.)  If for example, 22 of the rockets that fell in open areas were unsuccessfully engaged by Iron Dome, then its success rate would be 421/421+58+22) = 84.0%

An additional possible explanation is that the 84% figure is for all the Iron Dome engagements, including those before the most recent attacks.   In these earlier engagements, Iron Dome had a somewhat lower success rate of about 75% (see below).

 On the other hand, the official claim that less than 7% of the rockets hit populated areas is difficult to understand, since 7% of 1506 is about 105, far more than the 58 that actually hit urban areas.   (Unless the IDF is using “populated” and “urban” to describe different areas.)  On the other hand if we take the 7% to mean the percentage of rockets that would have hit populated areas in the absence of Iron Dome,  then the figure of 7% seems far too small (479/1506 = 32%) even allowing for the possibility that some of the intercepted rockets would have fallen outside of populated areas.

In my post of July 19 2012, I compiled claims of Iron Dome effectiveness from its first operational  use on April 7, 2011 through  March 2012.  Based on these, I estimated that Iron Dome had a success rate (measured per incoming rocket, not per interceptor fired) of about 75% against a total of about 120 rockets engaged. 

Thus Iron Dome’s claimed success rate of 84% in the latest attacks appears to be significantly better than its previously performance (this is more evident if one views the failure rate as decreasing from 25% to 16%).  This is not too surprising, given that you would expect the defense to learn from previous experiences and that some of the earlier failures were from correctable technical problems.  (Of course, the attacker can also learn, however, there is nothing in the publicly available data to suggest that Hamas took any steps to counter the defense, other than firing more rockets.)  Finally, it is important to bear in mind that all these figures are provided by the Israeli government, usually shortly after the attacks, and there is probably no way to independently confirm them.