Understanding Bankrate's rate averages
By Bankrate.com

Bankrate's site displays two sets of rates that are produced from two surveys we conduct: one daily and the other weekly. They're both useful, but they're samples of different groups and they serve different purposes.

You will see daily rate averages on the site in boxes labeled "overnight averages" (these calculations are run after the close of the business day).

Included there are rates we have collected on the previous day for a specific banking product. Overnight averages tend to be volatile. They help consumers see the movement of rates day to day. The institutions included in the "overnight averages" tables will be different from one day to the next, depending on which institutions' rates we gather on a particular day for presentation on the site.

The "National Index," or "national survey of large lenders," is conducted weekly. The results of this survey are quoted in our weekly articles, the Interest Rate Roundup and our Mortgage Analysis. To conduct the National Index survey, Bankrate obtains rate information from the 10 largest banks and thrifts in 10 large U.S. markets. We've conducted this survey in the same manner for almost 20 years, and because it's consistently done the way it is, it gives an accurate national apples-to-apples comparison.

The credit card index is based on a weekly survey of the 50 largest card issuers, ranked by total receivables, an industry standard measure.