1. For performing portfolio analysis, a method comprising: A) computing with a computer system including at least one computer, from portfolio data given by input signals that represent, for each of a sequence of record times, an actual portfolio's positions in respective assets, a plurality of values, each said value being: i) denominated an action value, ii) associated with a respective position and pair of record times, and iii) equal to the difference between: a) the respective position's portfolio weight on the later record time of the respective pair thereof and b) that position's portfolio weight on the earlier record time of that pair thereof adjusted for portfolio returns between that earlier record time and that later record time; B) computing with the computer system a performance measure from the action values thus identified; and C) generating with the computer system an output based on that performance measure.