If a customer does not give a broker his or her instructions, cost basis reporting on Form 1099-B for a stock holding where there have been multiple purchases at different times is done on a:

FIFO basis.

Explanation:

If a customer does not give a broker his or her instructions, cost basis reporting on Form 1099-B for a stock holding where there have been multiple purchases at different times is done on a first in, first out (FIFO) basis.

According to the internal revenue service (IRS), a Form 1099-B is a tax form which is received by individuals from their brokers or barter exchange enumerating their proceeds (gains) and losses from transactions made for a given tax year. The transactions include sales of stocks, forward contracts, derivatives, bonds, commodities, options, debt instruments for cash through a broker or barter exchange. It is necessary to also state on the Form 1099-B whether the holding period is short-term or long-term.

If the customer didn't give a broker his or her instructions and there have been multiple purchases of the stock at different times, it is reported or done on a first in, first out (FIFO) basis because the earliest or oldest unit of stock purchased are also the first unit that are sold.