Online retailers know that it’s common for a shopper to put items into a shopping cart and then leave without purchasing. Now, for the first time, the industry can track each day how the shopping cart abandonment rate varies across 50 e-commerce sites, giving retailers a way to benchmark their performance against others and to better time promotions aimed at convincing shoppers to return to purchase.

E-mail marketing service provider Listrak introduced its Shopping Cart Abandonment Index this week, providing a daily measure of the average abandonment rate among 50 of its top e-retailer clients. The index, accessible at www.Listrak.com/sca-index, allows a retailer to hover his mouse over each day to see the abandonment rate for that day.

The page also shows the average abandonment rate for the past six months—77%—and for today—73%.

“Retailers are losing tons of revenue in shoppers who begin the purchase online but then don’t complete it,” says Heather Bonura, director of brand strategy at Listrak. “Seeing trends when there is high abandonment may give some insight as to what offers to put in cart abandonment e-mail campaigns to recover those sales. That is, if it’s a time of really high abandonment, maybe they put a free shipping offer in the first or second cart abandonment reach-back e-mail message to close that sale. “

Free shipping offers do induce online shoppers to complete a purchase. A look at the index for the past six months shows shopping cart abandonment hit a low of 67.66% on Dec. 16, when some 2,300 online merchants participated in Free Shipping Day, eliminating shipping charges for that day. The second-lowest rate was on the Monday after Thanksgiving, or Cyber Monday, when the abandonment rate was 68.88% as many e-retailers heavily promoted discounts as well as free shipping.

Forrester Research Inc. last reported an industry average for shopping cart abandonment in 2010, when it put that rate at 55%. Forrester did not report a similar statistic for 2011.

Forrester typically surveys mostly larger retailers and consumer goods manufacturers. Listrak’s client base is more mid-market, with clients that include Vintage Tub and Bath, No. 411 in the Internet Retailer Top 500 Guide, and Jimmy Jazz Ecommerce LLC, No. No0. 746 in the Second 500 Guide.