In every test to date, the defense had detailed information in advance on every aspect of the test. This information includes the timing of the test, the target trajectory, and the intercept location. In addition, the defense knew in detail the anticipated appearance (i.e., the radar and infrared signatures) of the warhead and other objects. The defense then matched the measured signatures to the expected signatures of these objects to identify the warhead. As a result, these tests did not assess the ability of the defense to discriminate the warhead from other objects under conditions in which it does not have this a priori information.

The importance of this a priori information to the performance of the defense during these intercept tests is underscored by the reaction of Missile Defense Agency Director Lt. Gen. Ron Kadish during IFT-5. When he was told that the balloon decoy did not deploy properly, he responded "The decoy is not going to look exactly like what we expected. This presents a problem for the system that we didn't expect." The test later foundered when the kill vehicle failed to separate from its booster, so the test was not able to provide information on the extent to which the balloon's failure posed a problem to the defense.

There is no reason to assume that such a priori information will be available to the defense before an actual attack.