It is difficult to measure how often naked short selling occurs. Fails to deliver are not necessarily indicative of naked shorting, and can result from both "long" transactions (stock purchases) and short sales. Naked shorting can be invisible in a liquid market, as long as the short sale is eventually delivered to the buyer. However, if the covers are impossible to find, the trades fail. Fail reports are published regularly by the SEC, and a sudden rise in the number of fails-to-deliver will alert the SEC to the possibility of naked short selling. In some recent cases, it was claimed that the daily activity was larger than all of the available shares, which would normally be unlikely.