1. An uncertainty calculation system represented as a plurality of connected modules: wherein each module is associated with a set of inputs, one output and one function, in which the function is configured to determine the value of the output from the value of each input to the module (module-input) and to determine the contribution to the uncertainty in the value of the output arising from the uncertainty in the value of any input to the system (system-input); and wherein one module has an output that represents a measurement result and in which the contributions to the uncertainty are combined to determine the uncertainty in the measurement result.