The measurement function and all the effects that are sources of uncertainty to the retrieval process are presented diagrammatically in an uncertainty tree diagram. This diagram has the measurement function in the centre. Where this can be written algebraically it is expressed in full, where it is an iterative or look-up-table based process, it is presented as a function of input parameters. For each parameter within the equation we consider the origin of uncertainties associated with that quantity. The sensitivity coefficients, expressed as partial derivatives, relate an uncertainty in one quantity to the uncertainty in the calculated quantity. At the end of each “twig” of this “tree”, there are the effects. As with the FCDR (see D2-2a report) there sometimes needs to be a decision on how far each branch of the diagram should go. We have to balance the need for an intuitively understandable diagram, with the desire for a rigorous and full treatment. This compromise requires serious consideration.