Stale Prices: Empirical studies generally require a series of current prices, and, unless the price formation process is observed continuously, this is impossible to achieve and stale prices must be used.
If the aim is to produce a price series that is regularly spaced in clock time (say every 15 minutes), there may not be trades or quotes at these precise times, and the most recent price is usually used as a proxy for the current price, resulting in at least some stale price is usually used as a proxy for the current price, resulting in at least some stale price observations.
This is most helpful for ETFs that invest in domestic securities, however, because the INAV may reflect stale prices
of foreign securities in markets that are closed.
That means that the fund's current net asset value relies on stale prices
for 24 stocks out of a total of 28.
Unlike hedge funds, which as unregulated or loosely regulated entities selling mainly to sophisticated investors can afford to value a portfolio infrequently and even make use of stale prices
, mutual funds are regulated entities obliged to produce a daily NAV using the most accurate, clean and up-to-date prices available.
Asness, Krail, and Liew  argue that the presence of stale prices due to illiquidity or managed pricing can artificially reduce estimates of volatility and correlation with traditional indices.
They include self-selection bias, instant history or backfilling bias, survivorship bias, stale price bias, and multi-period sampling bias.
Other investors were allowed to conduct market timing trades to take advantage of stale prices
used by funds to calculate their net asset values at funds with stated policies against such trading.
Mutual fund prices become outdated, or "stale." Someone who can buy or sell to exploit the gap between stale prices
and real prices - a "market timer" - can make a killing.
Yet, clearly premiums exist and cannot be explained away as artifacts of stale prices
As a result any nontrading bias in the basis due to stale prices
should be small.