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We consider the complexity of approximating the partition function of the ferromagnetic Ising model with varying interaction energies and local external magnetic fields. Jerrum and Sinclair provided a fully polynomial randomised approximation scheme for the case in which the system is consistent in the sense that the local external fields all favour the same spin. We characterise the complexity of the general problem by showing that it is equivalent in complexity to the problem of approximatelydoi:10.1017/s096354830600767x fatcat:ebayzz6xgvduhk4mvfhgpdwgpm