Biases in hedge fund return data

It's well known that hedge fund indices are very upwards biased.  An article in the FT today talks in detail about these biases.  They stem from survivorship bias (only winners are in the return data), back fill bias (when successful funds that hadn't reported start reporting returns and back fill their data), and end of life bias (when failing funds stop reporting).   These biases can lead to indices being overstated by a massive amount.  A study of 2006 returns by Roger Ibbotson and Peng Chen reported that while the indices reported a return of 16%, the actual return was nearer 8%.  The FT quotes a hedge fund investor who states that "We adjust single manager indices down by 4 percent a year..."