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We use ion beam proximity lithography (IBPL) to produce 4 mm times 4 mm arrays of 220-nm dots in perpendicularly oriented Co/Pd multilayered media. A novel technique of overlapping neighbors is used with IBPL to generate samples with controllable size distribution sigmaD of the nanoarrays. The switching field distribution sigmaHcr/HCr is measured for each sample before and after irradiation, and a linear relationship is experimentally found between sigmaHcr/HCr and sigmaD. Empirical calculations support our determination that self-demagnetization fields are responsible for a portion of the switching field distribution. Using the zero intercept of the experimental data, we find the inherent anisotropy distribution in the Co/Pd samples is on the order of 10%. We further irradiate the samples with He+ ions to alter the anisotropy and better understand the role of shape anisotropy. As interface anisotropy is reduced in the Co/Pd bit-patterned medium (BPM) samples, shape anisotropy has a much greater contribution on the switching field distribution of the BPM samples.