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In the absence of any compelling physical model, cosmological systematics areoften misrepresented as statistical effects and the approach of marginalisingover extra nuisance systematic parameters is used to gauge the effect of thesystematic. In this article we argue that such an approach is risky at bestsince the key choice of function can have a large effect on the resultantcosmological errors. As an alternative we present a functional form fillingtechnique in which an unknown, residual, systematic is treated as such. Sincethe underlying function is unknown we evaluate the effect of every functionalform allowed by the information available (either a hard boundary or somedata). Using a simple toy model we introduce the formalism of functional formfilling. We show that parameter errors can be dramatically affected by thechoice of function in the case of marginalising over a systematic, but that incontrast the functional form filling approach is independent of the choice ofbasis set. We then apply the technique to cosmic shear shape measurementsystematics and show that a shear calibration bias of |m(z)|< 0.001(1+z)^0.7 isrequired for a future all-sky photometric survey to yield unbiased cosmologicalparameter constraints to percent accuracy. A module associated with the work inthis paper is available through the open source iCosmo code available athttp://www.icosmo.org .

Cosmological systematics beyond nuisance parameters: form‐filling functions