A change this week to Google’s formula for determining search rankings serves as a reminder to online retailers about the importance of including original content on their e-commerce sites.

Google doesn’t normally announce tweaks to its search ranking algorithm, and though this change merited a Google blog posting Thursday evening, the details are typically vague. But Google says the changes will affect nearly 12% of searches, and will push sites that the search engine considers higher quality closer to the top of search results.

“This update is designed to reduce rankings for low-quality sites—sites which are low value-add for users, copy content from other web sites or are just not very useful,” according to the blog posting from Amit Singhal, Google fellow, and Matt Cutts, a Google principal engineer.  “At the same time, it will provide better rankings for high-quality sites—sites with original content and information such as research, in-depth reports, thoughtful analysis and so on.”

Whether the change will immediately affect retailers’ search rankings is unclear. Indeed, at least one search expert, Byrne Hobart, marketing strategy director for Blue Fountain Media, an online marketing firm, doubts that retailers had to worry much. “In general, it’s not going to have a significant effect on retailers themselves,” he says, adding that with the changes, Google appears to be targeting content sites, not retail sites.

But, at the least, the change highlights again the importance that Google places on unique content for search rankings.  “Retailers that simply use the web copy from manufacturers or other resellers and don’t add anything new will have a harder time competing with all the other resellers that are doing the same thing,” says Mark Hawks, founder of SEOSavvy, a web marketing firm. “Unique content is very important for retailers and e-commerce.” Online textbook retailer eCampus.com moved up in natural search rankings by loading up its web site pages with more original content, including its own product descriptions and user reviews, vice president of marketing Mark Carson explained last week at the Internet Retailer Web Design & Usability Conference in Orlando.

The changes could benefit retailers indirectly, says Joshua Bixby, co-founder and president of Strangeloop, a company that helps web merchants improve site speed and rankings. “This is excellent news for e-retailers. The quality of Google results has dramatically decreased as content farms have learned to game the system,” he says. “I am ecstatic that companies focused on creating unique content and providing real value to customers will see search rankings improve.”

This week’s Google announcement comes about a month after the search engine said it would change its algorithm to reduce the amount of web spam, which refers to sites with little original content and irrelevant links.  Web spam can push down a legitimate site in search rankings, according to a report from search marketing firm Greenlight.