A variation of the survivorship bias that results from inclusion of a new hedge fund into a given index and its past performance is ‘backfilled’ into the index’s database. In other words, this bias occurs when the performance of a fund is added to database listings months or even years after inception. Funds joining a given database are often allowed to backfill their historical returns, therefore being added to the database with instant history, even though they were not included in the database in previous years. This causes another upward bias: new managers usually are entered into a database after a period of good performance, when entry seems particularly alluring. During periods of bad performance, fewer managers join databases, and therefore bad performance is rarely backfilled into the averages.

Backfill bias also known as inclusion bias, instant history bias or backfilling.