The study released by e-commerce rating company BizRate.com and The NPD Group Inc., a consumer panel research firm, was based on 10,000 respondents.

When asked why they are abandoning online shopping carts, 31 percent of respondents claimed they "simply changed their minds." Another 24 percent decided that the shipping and handling costs were too expensive.

What is happening to these lost sales? Thirty-nine percent of the survey respondents did not purchase the items at all. Another 26 percent purchased the product from a competitor, while 17 percent went off-line to get it.

The remaining 18 percent did return to the same site at a later time to complete the transaction.

The good news for online retailers is that 58 percent of the survey respondents stated they went back to the online store, and, of those people, 53 percent made purchases during subsequent visits. Furthermore, 74 percent of online buyers say they are very satisfied with their overall online buying experience.

Perhaps not surprisingly, the study found that the items most often abandoned are also the ones online consumers are buying most frequently: books, music, computer software, computer hardware and apparel.

Founded in 1996, BizRate.com is an unbiased, independent e-commerce rating guide built on the experience of millions of actual online buyers. NPD Group is an international marketing information company that specializes in tracking consumer purchasing and behavior.