The most significant concern with global formulary apportionment is the difficulty of implementing the system in a manner that both protects against double taxation and ensures single taxation. To achieve this would require substantial international coordination and consensus on the predetermined formulae to be used and on the composition of the group in question. For example, to avoid double taxation there would have to be common agreement to adopt the approach in the first instance, followed by agreement on the measurement of the global tax base of an MNE group, on the use of a common accounting system, on the factors that should be used to apportion the tax base among different jurisdictions (including non-member countries), and on how to measure and weight those factors. Reaching such agreement would be time-consuming and extremely difficult. It is far from clear that countries would be willing to agree to a universal formula.