A portfolio-analysis tool receives data that describe an actual portfolio. It computes from those data the returns or other performance measures of hypothetical portfolios whose holdings are drawn from the assets that the actual portfolio held during some period. Among the purposes of doing so is to detect biases made in investment-portfolio actions of the type taken, for instance, to accommodate cash inflows and withdrawals. For that purpose, differences between the hypothetical portfolio and the actual portfolio are so made as to offset portfolio actions identified by finding differences between the weights that positions actually exhibit and the weights they would result from return only. Returns for the hypothetical portfolio are computed by calculating an offset return incrementally, one such offset at a time, and then computing the hypothetical portfolio's return as the sum of quantities proportional to the offset return and that of the actual portfolio.