Online shopping is not always transparent and depending on your luck, you may end up paying more for the same product on the same site than what your friends spent, new research shows.

E-commerce websites manipulate data to make some shoppers pay more than others, the findings showed.

"Overall, we find numerous instances of price steering and discrimination on a variety of top e-commerce sites," the research revealed.

Using complex algorithms that crunch data from users' profiles and past activity, e-commerce websites manipulate search results or customise prices without the user's knowledge.

This study tracked searches on 16 popular e-commerce sites and found that six of these sites used some form of personalised pricing.

This kind of selective pricing is also much harder to detect on e-commerce sites."I get this question from people all the time: 'How do I get the best price?' The truth is I don't have a good answer," said study co-author Cristo Wilson, assistant professor at Northeastern University, the US.

"It changes depending on the site, and the algorithms they use change regularly. Good advice today might not be good advice tomorrow. The point is that as a consumer, you are at a disadvantage unless it's transparent," Wilson explained.

The findings will be presented in the 2014 Internet Measurement Conference in Vancouver, Canada, next month.