There are inherent biases in almost any database and separate accounts/collective investment trusts are no exception as it is a voluntary database.
There are three data biases that can affect the reported performance of separate accounts and collective investment trusts :
Survivorship bias or only including surviving investment vehicles, which leads to an upward bias in performance reporting, primarily affecting the category averages.
Selection bias: generally those separate accounts and collective investment trusts that are performing well have an incentive to report their results to a database in order to attract new investors. This again leads to an upward bias in overall database performance.
Catastrophe or liquidation bias: separate accounts and collective investment trusts that are performing poorly and likely to cease operations stop reporting their performance before they actually close shop. This results in an upward bias in overall database returns and a downward bias in risk.
It is also possible that those separate accounts and collective investment trusts that are very successful have little or no incentive to report their performance to a voluntary database because they have already attracted a sufficient number of investors. This would lead to a downward bias of separate account/collective investment trust performance reported by the databases.