There are conflicting accounts of the size of the subprime market. Depending on whom you talk to, it accounts for 20 percent of all mortgage loans, 15 percent or 13.5 percent.

Estimating the size of the subprime market is tricky for a number of reasons. For one, it's sometimes hard to distinguish between a subprime mortgage and an Alt-A loan -- a grade of mortgage between prime and subprime. For another, there are two ways to count them: by the number of loans or by total dollar value. Then there's the question of whether you're talking about all loans originated in a certain year or all outstanding mortgages.

Standard & Poors says subprime originations totaled $421 billion in 2006. The Mortgage Bankers Association says all originations totaled $2.5 trillion. If both data sources are accurate, that means 16.8 percent of mortgage volume consisted of subprime loans last year.

That's dollar volume, not the number of mortgages. Subprime mortgage balances are probably smaller than average, so more than 16.8 percent of borrowers got subprime loans.

These statistics rely on lenders to define what they mean by subprime, and different lenders have different definitions. As a rule of thumb, a subprime mortgage is a home loan to someone with a credit score below 620. But some lenders count loans as subprime even if the borrowers have credit scores of 660 or higher, if the borrower makes a down payment of less than 5 percent or does not document income or assets. Other lenders might count those loans as Alt-A.

There isn't a definition of subprime that everyone agrees on. That's partly what makes it difficult to judge the size of the subprime market.