Nuclear data files are normally based on statistical analyses and evaluations of all relevant measured data reported in the open literature. Unfortunately, there may be omissions in the resulting data libraries that need to be filled to avoid serious impact on the ability to undertake the desired calculations with confidence. Setting this issue aside until Sections 4 and 5, some consideration is given below to data uncertainties and the value of sensitivity studies in identifying the main parameters to be improved in order to increase the accuracy and reliability of decay-heat calculations.