Editing Hedge fund (section)

====Non-investable indices====
Non-investable indices are indicative in nature and aim to represent the performance of some database of hedge funds using some measure such as mean, median, or weighted mean from a hedge fund database. The databases have diverse selection criteria and methods of construction, and no single database captures all funds. This leads to significant differences in reported performance between different indices.

Although they aim to be representative, non-investable indices suffer from a lengthy and largely unavoidable list of [[Selection bias|biases]]. Funds' participation in a database is voluntary, leading to [[self-selection bias]] because those funds that choose to report may not be typical of funds as a whole. For example, some do not report because of poor results or because they have already reached their target size and do not wish to raise further money.

The short lifetimes of many hedge funds mean that there are many new entrants and many departures each year, which raises the problem of [[Attrition bias|survivorship bias]]. If we examine only funds that have survived to the present, we will overestimate past returns because many of the worst-performing funds have not survived, and the observed association between fund youth and fund performance suggests that this bias may be substantial.

When a fund is added to a database for the first time, all or part of its historical data is recorded ex-post in the database. It is likely that funds only publish their results when they are favorable, so that the average performances displayed by the funds during their incubation period are inflated. This is known as "instant history bias" or "backfill bias".