There is also an older approach, based on a seminal paper by Nordheim in 1928, that formulates the problem differently and then uses first K p and then ε n (or a related quantity) as the variables of integration: this is known as "integrating via the normal-energy distribution". This approach continues to be used by some authors. Although it has some advantages, particularly when discussing resonance phenomena, it requires integration of the Fermi–Dirac distribution function in the first stage of integration: for non-free-electron-like electronic band-structures this can lead to very complex and error-prone mathematics (as in the work of Stratton on semiconductors). Further, integrating via the normal-energy distribution does not generate experimentally measured electron energy distributions.