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What should my cookie length be?

Cookie length is the amount of time that a cookie will remain on a user’s PC after they have clicked an affiliate link to a merchant’s site, unless it is overwritten by another affiliate’s cookie.
Owen Hewitson
Affiliate Window & buy.at
Fri, 25-03-2011 09:43

Because clicks are an imperfect measure of the influence an affiliate has over a user’s decision to buy, the fact that there is a cookie present is not proof that a customer’s decision was made on the basis of visiting the affiliate site and clicking through to the merchant. Nevertheless, after examining data from sales tracked via the Affiliate Window network, we found the vast majority of affiliate sales occur within 24 hours of the cookie being placed.

Knowing this, how should advertisers set their cookie length?

On one side is the argument that you can reduce your cookie length on the grounds that most affiliate sales will be captured regardless. But there again, is this not just as much an argument for extending them?

Some large programmes, most notably Amazon, use a 24 hour cookie. But is it really fair that if an affiliate provides the deciding factor in the user’s decision to buy, that unless they do so within 24 hours the affiliate gets nothing?

The same question can be asked, to an even greater extent, of programmes that only offer single-session cookies. In recognition of this, the industry standard in the UK has always been 30 days, and often this is accepted by default.

Moreover, some advertisers choose the maximum cookie length, 999 days, as a gesture to affiliates that they are on their side - after all, many people do not keep the same computer for this long! - with the knowledge that they will rarely pay out on anything older than 30 days. If however they do, there is an argument that this customer can only be said to have a very tenuous link to the influence that the affiliate has played. This is why some advertisers reject long cookie periods on principle.

Ultimately though, we cannot know everything about a user’s decision-making process, and who should be correctly attributed the sale. After all we cannot cookie the user themselves, only their devices, of which there are a growing number - be it their home or work PCs, their mobiles, or tablet devices.

As we noted above, the fact that the affiliate’s was the last cookie present does not tell us anything about whether or not, or to what extent, they successfully persuaded the customer to purchase. Commissions paid out on cookies older than 30 days only show that that affiliate was recognised as the last referrer by the merchant before the customer transacted. But if the advertiser is happy to reward the affiliate on the basis that no other actors can be attributed the sale, then so be it: the affiliate should get the commission.

So despite the debate, the final decision is therefore the advertiser’s, and affiliates can accept it or vote with their feet by choosing to promote a competitor.

In my opinion there are three factors which should be considered when deciding on cookie length. Watch out for my next article, which will be uploaded in the next few days, to discover what they are.

Owen Hewitson is a Client Strategist at Digital Window. He works with merchants on both the Affiliate Window and buy.at platforms helping to build and optimise their affiliate campaigns. Digital Window is the holding company for Affiliate Window and buy.at, two of the UK's leading affiliate marketing networks.

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What about repeat sales
emmahaslam's picture

One area that always seems to be overlooked regarding cookies is repeat business from customers affiliates have generated. Surely if a merchant can acquire customers who spend money more than once that's great news and setting up a cookie length that encourages affiliates to send more of them is worth considering.

Maybe if a merchant can marry up repeat customers to specific affiliates and the average lifespan of these customers then it would be great if they could tailor a bespoke cookie length to these affiliates rather than a one size fits all approach. Is this something networks are or should be looking into?