NEW YORK >> It may be a while, if ever, before investors can buy an exchange-traded fund made up of bitcoin and other digital currencies.

The Securities and Exchange Commission this week sent a letter to investment trade groups containing more than 30 questions that must be answered before a digital currency fund gets approval.

Among the concerns: How would funds calculate a fair value for volatile digital currencies, and what steps would funds take to ensure that investors can easily cash out their shares?

An exchange-traded fund, or ETF, is similar to a traditional mutual fund, except that investors can buy and sell it throughout the trading day. A digital currency ETF would allow mom-and-pop investors to own bitcoin and others digital currencies without going onto the private exchanges they trade on.