I view IRR as the return to the GP. In order to take into account Buffett's comment on holding assets in reserve for the eventual funding, I've been using Modified IRR to calculate the return to the LP. This encompasses the entire commitment period, capital calls and more importantly lack of capital calls. I know PME does this too, but I find MIRR easier to use and still reflective of the returns to the LP. Most full fund period MIRR returns are 200bps to 400bps' less than XIRR's. This also takes into account the "reinvestment risk" the LP's have when GP's return money quickly