Sentence:  This assumption is in the Raiffa and Schlaifer (1961, p. 72) spirit of using ‘a little ingenuity. . . to find a <WSD>tractable</WSD> function' to quantify risk-preferences and probability judgments so as to make the analysis feasible. 
Meaning of the word marked with <WSD> tags:  Sufficiently operationalizable or useful to allow a mathematical calculation to proceed toward a solution.