Probability of Kill, or Pk , is a value used by planners when setting up targeting. It refers to the probability (expressed as a fraction) that a hit by a particular weapon will actually destroy a specific type of target. For example, if ten hits on ten different targets of the same type results in eight kills, then that weapon has a Pk of 0.8 against that target type. This should not to be confused with the probability of hit, which is the probability in each case that the weapon will strike within its lethal radius of the target.

Update: I've gone and confoozled everyone including me.
Okay. Since noding Damage Expectancy I've realized that some of my studies and some of my sources occasionally conflate these two terms, and as a result, so have I. Let me try to fix this. In most simple cases, especially those involving conventional weapons and multiple attacks, the effects of weapon reliability are small enough to be ignored. Nuclear weapons are different because, well, nobody has ever used those weapons before. Sometimes we haven't even tested that particular system in its entirety; if it was developed after the Atmospheric Test Ban treaty, then we certainly haven't. So there's a good chance things just don't work.

Conventional weapons, however, are quite familiar, and we drop 'em all the time. Training drops count. So we can be relatively sure our estimates of their reliability are correct; furthermore, their reliability is pretty darn high.

The only difference: sometimes people will talk about 'single-shot Pk' which is, essentially, the same thing as DE without all those troublesome defenses and reliability issues; and, to make matters worse, they'll sometimes just say 'Pk' and expect you to figure it out.

There you have it. If you're being rigorous, a strike has a Pk; it consists of one or more attacks made by individual weapons or submunitions, each of which has a damage expectancy. Those DE numbers are rolled together to produce the Pk for the entire attack. Determining the full strike Pk can be done in the following way. If we define SPk to be strike Pk and Pk to be an individual attack (say, one of the N number of bombs assigned to a strike) then:

SPk = 1 - (1 - Pk)N

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