Deposited thin ferromagnetic films with uniaxial anisotropy have local regions with admixed biaxial anisotropy, although there is no biaxial component to the average anisotropy field of the whole film. Two methods are presented for measuring the biaxial contribution to the inhomogeneity in the anisotropy field. The effect of this biaxial inhomogeneity is investigated theoretically and found to explain much puzzling data: the various negative anisotropy effects, the relationship between α90 and Δ90 (the inhomogeneity in orientation and in magnitude of the anisotropy, respectively), the internal biasing field, the compositional dependence of the inhomogeneity and negative anisotropy, the deposition substrate temperature dependence of negative anisotropy, the position of the negative anisotropy peak, the shape of the probability density function for films with both large and small inhomogeneities, the ratio of positive to negative anisotropy, and the skew of the Δ90 probability density curves. Theoretical switching threshold curves for complex (biaxial) anisotropy are drawn; theoretical probability density functions are plotted for various values of biaxial inhomogeneity and compared with experimental curves. Local regions of biaxial anisotropy can arise from the interaction of strongly coupled adjacent areas of the film with different anisotropy fields, as well as from magnetocrystalline anisotropy.