 The two-slit experiment is a paradigm example of an interference experiment. One repeatedly sends electrons or other particles through a screen with two narrow slits, the electrons impinge upon a second screen, and we ask for the probability distribution of detections over the surface of the screen. One might naively try to calculate them by summing over the probabilities of detection at the slits multiplied by the probabilities for detection at the screen conditional on detection at the slits. But these are the correct probabilities for a different experiment, with detections at the slits, whether or not we believe that measurements are related to a ‘true’ collapse of the wave function (i.e. that only one of the components survives the measurement and proceeds to hit the  screen[2]).  If there are no such detections, in general there is an additional so-called interference term in the correct expression for the probability, and this term depends on both the wave components that pass through the  slits.[3] 