1. For performing portfolio analysis, a method comprising employing a computer system to: A) 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, identify, as relevant actions in respective said positions on respective record times, differences meeting a set of at least one criterion between: i) those positions' portfolio weights on those record times; and ii) those positions' weights on previous record times adjusted for the portfolio's positions' returns; B) compute a performance measure from the relevant actions thus identified; and C) generate an output based on that performance measure.