PSF investors - Facebook, EMC, Compal, page-917

It is your choice given all the shares have the same value at time of sale. For example say you have 1000 shares of company Y and paid $1 per share and later averaged down and bought a 1000 more at .50 cents, then sell a 1000 at .75 cents, it is up to you to choose whether to take a profit or loss, depending on what suits tax-wise. You could sell 1500 if you like, but you need to work out the average cost per share for tax.