Why do ads follow me around the web?
I feel like I'm being followed around the web, every site I visit shows me ads related to sites I've visited previously.
I'm worried about my online security and privacy: how much do they know about me and do these ads have access to my computer?
You're not going crazy: many sites follow their users and show them ads to persuade them to come back, ideally with cash.
It's called 'site retargeting' and it can be creepy or, if you're bombarded with inappropriate ads, annoying.
However, sites track previous users anonymously so it's not as big a breach of privacy as you might think, although some still find it a step too far.
If you're still on the fence read on to find out how these ads work and how it affects privacy.
Or, if you already know which side of the fence you're on, go ahead to find out how to stop behavioural ads in their tracks.
How ads follow you
Here's how it works: when you visit a site that wants to show you an ad later, they 'tag' your browser with a small text file called a cookie.
When you move on to a new site that has a space for personalised ads, even one not at all related to the original site, the page will recognise your tracking cookie and show you a 'relevant' ad.
If the site is lucky, the ad reminds you to return.
Maybe you decided those shoes were too expensive on Monday, for example, but you might buy them when you're feeling flush on Friday... if you see them again.
Following across devices
Cookies are linked to a browser - Internet Explorer, Firefox, Safari, Chrome, whatever you use to get online - and you can delete them.
But it's worth knowing that the way companies serve up targeted ads is growing ever more sophisticated: it's even possible for them to 'follow' users as they use the internet on different devices.
Now, if you were searching for some shoes at work, you might see the same pair when you search at home.
Companies do this by a process they call 'triangulation'.
They look for users with very similar patterns of use and conclude that the very similar ones are likely to be the same people. Then they show them 'relevant' ads.
In this case, actually the advertiser isn't following an individual, then, but more or less guessing that very similar searchers will be interested in the same things.
In fact, site retargeting is just one tool in online advertisers' arsenal.
Similar but different: search retargeting
For example, these site retargeting ads are very similar to a more familiar form of advertising online: search retargeting.
Google Adwords serves up adverts, on the side of your search and on other sites, based on the search terms you use or have previously used.
Other companies also target ads this way by buying the search data (the terms visitors use to access a site) from individual websites so, often, when you're seeing a retargeted ad the advertiser found you through your search term, just as Google advertisers do whenever you search there.
Search retargeting is less obvious than site retargeted adverts because the advertisers will likely be new to you but the principal is more or less the same.
Are cookies creepy?
Cookies aren't just used for advertising. They're the way sites remember our passwords, store items in a shopping basket and numerous other useful and non-invasive jobs.
Advertisers maintain that, when they explain how targeted adverts work, most people aren't bothered by them, or aren't bothered enough to stop them.
88.8% of participants in a July 2013 Toluna survey felt either neutral or positive about targeted ads and 42% of those surveyed rarely noticed them.
Problem 1: inappropriate advertising
If you're reading this article, you probably don't feel that way and that's most likely because you've had to endure inappropriate ads.
Advertisers can leave a gap between you viewing a site and seeing adverts for it but many don't seem to.
Advertisers can also set a maximum cap on the number of retargeted ads they show you, but again this isn't a rule.
Advertisers should also stop ads if you buy something but some don't and, if you end up buying elsewhere, then, well, they can't.
Mostly, sites just seem to go overboard - they're everywhere! - that's usually because the advertiser is getting paid by the click and they've gone at it head on.
It's worth pausing at this point, however, to point out that the alternative to targeted ads is untargeted ads, which may be even more annoying.
I don't know about you but I don't want to find out how a woman in [my current location] beat wrinkles or makes £500 an hour working from home.
I'll take the excessive shoes.
If annoying ads made you notice you were being tracked, however, that's not to say that being tracked isn't, in itself, somewhat concerning.
Problem 2: what do they know?
Lets be clear, cookies can't 'store' any information about you - your name, address or other identification - because they're really small text files, so there's no way that viruses or spyware could pick it up.
They also can't access data on your computer, they're just at work whenever pages load online and can only store information the page knows already, such as the page you're viewing and the page you came from.
However, when you go shopping online and a cookie - or, far more likely, cookies - track you they can build up a fair bit of information.
To put it another way, companies that you probably didn't give express permission to do so are recording your behaviour as you move around the web, using that data to create anonymous profiles.
Yeah, creepy's one word for that.
How to stop tailored ads
If it bothers you, there are a number of ways you can either prevent or stop advertisers following you around the web.
Stop an annoying ad
If you want to stop one or two particularly annoying advertisers in their tracks it's easy to do.
Go into your browser's settings and select the clear cookies option. Be aware that this will log you out of some sites.
Prevent annoying ads
There are a number of things you can do to prevent annoying ads appearing in the future.
1. Opt-out of behavioural advertising
Many ads now use the "AdChoices" icon, a little blue arrow in the corner of a banner ad.
AdChoices is a self-regulatory trade body that most of the biggest advertisers have signed up to: Amazon, Yahoo, Microsoft and Google are all participants of AdChoices.
The purpose of AdChoices is to provide consumers information on behavioural advertising and options to opt-out, either of single ads or from entire advertising companies.
Participating ads provide an 'X' icon you can click on the banner to remove that particular ad, or click through for further information for wider opt-outs.
2. Use private browsing or turn on 'do not track'
Your browser's private browsing mode will stop sites from planting cookies on you as you move around the web.
However, that means no useful cookies either (some sites won't let you sign up without cookies enabled) and no access to your personal browsing history.
Similarly, all the major browsers now have a 'do not track' option available in settings.
It's usually hidden in a long list and few people use it: Firefox say that, as of December 2012, 90% of their users had never touched it.
'Do not track' is the online equivalent of those little 'no junk mail' signs people put on their letterboxes, however, pages see the 'no cookies please' message but it's up to them to stop it.
3. Block all ads
A more blanket approach might be one of the web's numerous ad-blocking services.
Here are three of the best.
- Ghostery: we like Ghostery because it's really simple. As you browse around it shows you all the cookies tracking you in a little purple box - as you'll be unsurprised to learn at this point, there's often a lot - and you can go through and disable the one's you'd rather not have or, probably easier, enable the useful ones.
- Adblock plus: Install Adblock and enable 'disable tracking' for the most effective ad block around. The software also blocks banners, pop-ups and other online aggravations.
- Do not track plus: Do Not Track plus, which is similar to Ghostery, might also be worth checking out.
3. Log on, opt out
You may have noticed that websites must now inform users about any cookies and provide an opt-out.
It's a European Union law that means websites must now get implied consent to use non-essential cookies.
Before it existed, using a website was considered consent for tracking, now websites must at least tell users if any non-essential cookies are being used.
Implied consent typically means either the user clicks a 'yes I agree to cookies' button or they continue to use a site after a message about cookies has been delivered.
For example, anyone using our site for the first time is informed about cookie use and can view more information about the cookies we have present on the site over here.
However, note that the rules don't apply to all cookies: cookies that are necessary for the site to function - for online security or to put items in a shopping basket - can run even without consent.