1. A portfolio-analysis system comprising: at least one computer configured to: A) receive inputs representing portfolio data that specify, for each of a sequence of record times, an actual portfolio's positions in respective assets; and B) respond to a request for an indication of advantage by: i) identifying relevant actions on respective said positions in the portfolio data; ii) determining for a performance measure the values thereof exhibited by the actual portfolio and at least one hypothetical portfolio whose positions result from so modulating the actions of the actual portfolio that the average magnitude of the differences between the holding periods of the hypothetical portfolio's positions and those holding periods' average is less than that of the actual portfolio; and iii) generating an output based on a comparison of the values thereby determined for the actual portfolio and at least one said hypothetical portfolio.