The missile knows where it is at all times. It knows this because it knows where it isn’t. By subtracting where it is from where it isn’t (or where it isn’t from where it is) it obtains a difference or deviation. The guidance system uses deviations to generate corrective commands to drive the missile from the position where it is, to the position where it isn’t. The missile arrives at the position where it wasn’t, consequently the position where it was, is now the position where it isn’t. In the event that the position where it is now, is not the same position where it originally wasn’t, the system has acquired a variation, the variation being the difference between where the missile is and where the missile wasn’t. Moreover, the missile must now know where it was also. The “thought process” of the missile is as follows: Because a variation has modified some of the information which the missile had obtained, it is not sure where it is. However, it is sure where it isn’t and it knows where it was. It now subtracts where it should be from where it wasn’t (or vice versa) and by differentiating this from the algebraic difference between where it shouldn’t be and where it was, it is able to obtain the difference between it’s deviation and it’s variation, this difference being called the Error.

In the end, is CPSIA is the missile and Julie is the Error? Ah, that could explain why she’s the former CPSC spokesperson.