The Securities and Exchange Commission has written to Groupon asking the company behind the popular group discount site to clarify the accounting measures it used in its filing for an initial public offering.

In its regulatory filing last month, the Chicago-based online coupon provider referred to a measure called “adjusted consolidated segment operating income,” or “adjusted CSOI,” to measure its profitability (see Groupon’s Accounting Called into Question). The measure leaves out the company’s heavy marketing expenses as well as the expense of buying sites such as CitySearch in Europe. The Securities and Exchange is now looking into the matter, according to CNBC, and has reportedly asked the company to explain exactly what adjusted CSOI involves.

The SEC inquiry could delay Groupon’s IPO, which was anticipated to occur this fall. The IPO was expected to raise up to $1 billion, giving the company a valuation of as much as $10 billion.

However, the non-standard profitability measure has generated skepticism. The company claimed $81.6 million in adjusted CSOI during Q1 2011, but with marketing costs added in, the number would fall to a loss of $98 million, according to The Wall Street Journal.

Another measure that the company included in its S-1 registration documents was called “gross profits,” and referred to the part of a sale that the company would keep after paying off the merchant. However, that measure too would leave out the marketing and administrative expenses.

Groupon has already needed to amend its original registration statement once. The day after it filed for an IPO, its chairman said the company would be “wildly profitable,” violating the quiet period that companies filing for an IPO are expected to observe.