Cookie tracking for profit and pleasure

It is great to make sales. But you really need to know where these sales are coming from to optimise your marketing. A simple and effective way to do this is through cookie tracking. The basic process is:

  • A visitor arrives at a web page on your site.
  • A script on your web page stores a small file (cookie) on their computer with some tracking details, e.g. the web page they came from, the date they arrived and the page they arrived at.
  • As they navigate to other pages the Javascript on these pages recognises that the cookie already exists and doesn’t modify it.
  • When (if) the visitor makes a purchase, the contents of the cookie are sent through to your payment provider.
  • Your payment provider sends back the cookie data with all the other information about the sale.

From the referrer you can find out what your customer typed into a search engine to find you. For example if the referrer is: software

You can infer that the purchaser found you by typing “backup software” into Google. This is incredibly useful information. Once you have amassed enough of it you can find out which keywords are most effective at selling your product. For example, whether “back-up software” makes more sales than “backup software” or “back-up programs”. This can be very helpful for fine-tuning your marketing message, SEO and PPC campaigns. You can also find out which websites purchasers are being referred from, and even how long purchasers take to make a sale after first arriving at your site.

You can get a lot of this information from Google Adwords conversion tracking. But you will only get data on sales through Adwords. I want data on all my sales. You can also get some of this information through Google Analytics. But you can only get the information in the form that Analytics wants you to have it and the price is allowing Google to see all this data as well. So I think it is well worth doing your own tracking, even if you are using Adwords conversion tracking and Analytics.

If you do use tracking cookies you will find that there is no cookie data for many transactions or the cookie data is unreliable. Reasons for this include:

  1. The cookie has expired before the customer made the purchase.
  2. The cookie has been pushed out of the cache by other cookies. Browsers only have a limited cookie cache, and your cookie might be pushed out of the cache by others long before any expiration date you set.
  3. A different person is buying the software to the person who first arrived at your site.
  4. A different computer or browser is used to buy the software to the one use to find the site.
  5. The customer clicked a button in your desktop software (not a browser) to go to your site, so there is no referrer information.
  6. A firewall or other software is blocking cookies.
  7. The customer has disabled JavaScript in their browser.

So cookie tracking data is never going to be particularly reliable. My own data shows that about 30% of sales don’t return cookie data. But it is likely to be considerably worse for B2B sales due to the longer sales cycles and the increased likelihood of the buyer not be being the person who first found the product.

With these caveats in mind, I think it is worth the time to set up cookie tracking. It is pretty quick and easy to do. You can even use the free JavaScript published at Note the conditions of use. Note also what an ugly language JavaScript is[1]. I recommend placing the JavaScript in a single file which you include in each page, so you only have a single place to make modifications, for example:

<script language=“JavaScript” type=“text/javascript” src=“refercookie.js”> </script>

Sending the contents of the cookie to your payment provider is also quite straightforward. For example, for e-junkie I just use some JavaScript to extract the cookie contents and append:

&custom=<cookie contents>

to the end of the ‘Buy now’ button URL e-junkie gives you. The cookie data then comes back to me in the ‘custom:’ field of the e-junkie sale confirmation email (I believe all the major e-commerce providers support something similar). I then store the cookie data along with all the other sales data. I can use this data to generate various graphs and reports, including top-selling keywords and a graph of the time-taken to purchase. Unlike much of the data you get from Analytics this is data you can really use, e.g. for the top selling keywords:

  • Make sure they are in your Adwords campaign.
  • Write additional content pages based around these keywords to attract targeted traffic.
  • Consider including these keywords in the strapline on your home page.

The use of cookies does have privacy implications, but these are often overstated. In theory all the information in a cookie could be retrieved from server log files, cookies are just a more convenient way or doing it. Users can also disable cookies in their browser settings or using other software. I think it is fine to use cookies as long as you make this clear to your visitors. You should still have a clearly stated privacy policy for your website and this should contain a brief description of what information you are storing in cookies.

Knowing a bit about cookies can also help you as a consumer. A while back I was interested in buying a large VDU from Dell. I browsed around their site and found a good deal. I went back some time later to buy the monitor after I had bought a new PC, but the price had gone up considerably. On a hunch I deleted Dell’s cookie and refreshed the page. The price dropped back to the original price. I believe that Dell knew from a cookie that:

  1. I had logged in as a business user; and
  2. Had just purchased a new PC from Dell.

Consequently they expected me to be less price sensitive than a consumer shopping for just a VDU and upped the price. I can’t prove this. It is also possible (but unlikely) that they just happened to drop the price in the few seconds before I did a refresh. Anyway, try it next time you want to buy something expensive online. Note that it might be easier to use another browser (e.g. Opera or Safari) than to delete cookies. Let me know if you get a similar result.

[1] It has been said that JavaScript bears as much resemblance to Java as the Taj Mahal Indian restaurant bears to the Taj Mahal. And Java is hardly a ‘looker’.

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