42In real experiments very often an upper bound for the uncertainty of one of the conjugate variables, say x  , is given - as a property of the measuring device (slit). The corresponding Δx  is identified with the single-particle uncertainty - but this is a very rough estimate which does not do justice to the universal validity of the IUP claimed by its inventors. In order to know the real Δxi  for particle i  one should be able to measure the position of particle i  within the slit - and no such experiment has ever been performed