Many studies have been constrained by the lack of suitably long time series, and have resorted to estimation devices that almost surely have distorted the findings—such as imposing restrictions on the lag distribution length and shape or creating estimates of past data using crude extrapolations from the present, a data step that is not always apparent to the reader of the distilled research product. The two principal areas of difficulty are (a) in identifying the component of productivity growth that is attributable to research-induced changes in knowledge and then further attributing responsibility among alternative public and private providers of R&D (the spatial and institutional-cum-sectoral attribution problem), and (b) in identifying the research lag structure (the temporal attribution problem). Similar problems arise when the analysis is focused on a particular innovation or applied to all research undertaken by a national system, but the specifics differ as does the potential severity of the problems.