see also huffman and evenson alston craig and pardey discussed the issue of knowledge depreciation drawing on the previous literature and these arguments are restated and refined by alston pardey and ruttan and alston andersen james and pardey after dropping some outliers and incomplete observations they conducted regression analysis using a sample of estimates with a mean of a mode of and a median of they found results that were generally consistent with expectations but in many cases they could not distinguish statistically significant effects on the estimated rates of return associated with the nature of the research being evaluated the industry to which it applied or the evaluation methodology because the signaltonoise ratio was too low nevertheless a predominant and persistent finding across the studies was that the rate of return was quite large the main mass of the distribution of internal rates of return reported in the literature is between and per annum