 Second, a single weak measurement corresponding to a weak interaction between particle and probe will not yield much information about the particle, and hence it is usually assumed that such a measurement will be repeated a large number of times, with the particle prepared in the same initial state for each run, so that useful information can be extracted from the random outcomes. It is sometimes assumed that because the interaction is weak, in none of the runs will the state of the particle change significantly. However, that is not necessarily the case. Weakness of the interaction is also consistent with the particle’s state being drastically altered in a number of runs, provided these are but a small fraction of the total. A useful example is provided by Feynman (Feynman, Leighton, &amp; Sands: Chapter 1) in his discussion of a weak light source used to determine which of two holes an electron passes through on its way to a distant screen where successive arrivals display a two-hole (double slit) interference pattern. If the light source is very weak, only for a small fraction of the runs does a scattered photon reveals which hole the electron went through. But if one collects data for these rare instances one finds a very large effect: in these runs the interference pattern is completely washed out.