Hedge fund performance data suffers from a number of biases, primarily relating to the lack of publicly released fund valuations. As such, any index reporting, hedge fund performance data is relying on the data hedge funds choose to disclose to the index providers. The first bias this leads to is selection bias, which relates to the fact that some hedge fund managers may choose to only report data to index providers in relation to the well performing funds, and may choose not to submit data on poorly performing funds. This will lead to an upward bias in the index's calculation of the performance of the hedge fund industry. The selectivity of reporting may also lead to backfill bias, which results from historic fund performance being included in an index. When a fund first reports performance data to that index, the fund is likely to have been in existence for a number of years and will only be reporting data to an index if that performance is good. This will result in an improvement in the historic performance of the index once this data is included, The final potential bias is survivorship bias, which relates to the historic performance of funds, which cease reporting to an index being removed from that index. It is far more likely that funds which stop reporting performance to an index are the poorly performing funds, which will also improve the historic reported performance of the index. Although index providers attempt to have controls to protect for these biases, they still remain due to the private nature of hedge funds.