Editing Hedge fund (section)

===Hedge fund indices===
[[Stock market index|Indices]] play a central and unambiguous role in traditional asset markets, where they are widely accepted as representative of their underlying portfolios. Equity and debt [[index fund]] products provide investable access to most [[developed market]]s in these asset classes.

Hedge fund indices are more problematic. The typical hedge fund is not traded on exchange, will accept investments only at the discretion of the manager, and does not have an obligation to publish returns. Despite these challenges, Non-investable, Investable, and Clone indices have been developed.

====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".

====Investable indices====
Investable indices are an attempt to reduce these problems by ensuring that the return of the index is available to shareholders. To create an investable index, the index provider selects funds and develops structured products or derivative instruments that deliver the performance of the index. When investors buy these products the index provider makes the investments in the underlying funds, making an investable index similar in some ways to a fund of hedge funds portfolio.

To make the index investable, hedge funds must agree to accept investments on the terms given by the constructor. To make the index liquid, these terms must include provisions for redemptions that some managers may consider too onerous to be acceptable. This means that investable indices do not represent the total universe of hedge funds. Most seriously, they under-represent more successful managers, who typically refuse to accept such investment protocols.

====Hedge fund replication====
The most recent addition to the field approaches the problem in a different manner. Instead of reflecting the performance of actual hedge funds, they take a statistical approach to the analysis of historic hedge fund returns and use this to construct a model of how hedge fund returns respond to the movements of various investable financial assets. This model is then used to construct an investable portfolio of those assets. This makes the index investable, and in principle, they can be as representative as the hedge fund database from which they were constructed. However, these clone indices rely on a statistical modelling process. Such indices have too short a history to state whether this approach will be considered successful.