Elizabeth Weise

USATODAY

SAN FRANCISCO— You’re not paranoid, you really are being followed more online.

A study that looked at web tracking over the last 20 years found that at least 75% of the world's 500 most popular websites contain web trackers, up from fewer than 5% in 1998.

“The number of trackers have increased, the ability of the top trackers to track you across sites has increased and the complexity of the trackers has increased,” said Adam Lerner, a security and privacy researcher at the University of Washington in Seattle and one of the study designers.

The researchers used a unique Internet archive, the Wayback Machine, to look at third-party tracking code used on web sites over time. These-third party trackers first appeared in 1998 and their use has been rising almost continuously ever since.

“Obviously they have legitimate purposes, but they also have privacy implications,” said Franziska Roesner, professor of computer science at the University of Washington and an author on the paper, which was presented last week at a computer conference in Austin, Texas.

Half of the top 500 websites they looked at contained at least four of the third-party trackers, the researchers found.

And it’s not simply those individual companies that are tracking users, said Ali Lange, a policy analyst with the Center for Democracy and Technology in Washington D.C.

“It’s important to remember that those four brokers are then selling that data to others, so it’s much broader,” she said.

The rise of web tracking and more targeted advertising has helped fund the explosion of online content and build such web behemoths such as Google, Facebook, Amazon and others. It's even part of the reason Verizon recently bought Yahoo, to bolster its potential audience for ads. It's also led to more consumers installing ad-blocking software, which resulted in pushback from companies like Facebook to thwart it.

The most commonly-found ad server, Google-owned DoubleClick, appeared on over 15% of all the sites. Other third-party trackers the researchers found included Scorecard Research and Quantcast.

The researchers also found that Google analytics, a Google program that allows the owner of a webpage to analyze traffic to it, was on more than 30% of sites. However, the program doesn't generally track information about users for ad use, the researchers noted.

How they work

Websites today commonly include tracking code from third parties, such as advertisers, social media sites and website analytics services.

In the simplest tracking mechanism, those third parties store a "cookie" containing a unique identifier (such as "USER1234") in the user's browser.

Whenever a user visits a website containing code from one of those third party trackers, that code is used to look for the user’s unique identifier stored in the user’s browser.

If it finds the unique identifier, it sends information about what the user viewed to its collection of information about them out to the company that placed it.

Thus, the third party trackers are able to build "browsing profiles" of users. While these don't include their name, they do include information of interest to advertisers. For example, a profile might show that USER1234 frequently visits sites about guitars, probably lives in Kansas, has looked at ads for amplifiers, buys dog food from a specialty site and has searched for information about cement shingles.

“They can create a pretty fine-grained view of who you are and what you’re interested in,” said Lerner.

It’s these trackers that result in “those annoying ads following you around and kind of creeping you out,” said Cooper Quintin, a staff technologist with the Electronic Frontier Foundation, a digital rights group in San Francisco.