In the case of cost per mille/click, the publisher is not concerned about a visitor being a member of the audience that the advertiser tries to attract and is able to convert, because at this point the publisher has already earned his commission. This leaves the greater, and, in case of cost per mille, the full risk and loss (if the visitor can not be converted) to the advertiser.
Cost per action/sale methods require that referred visitors do more than visit the advertiser's website before the affiliate receives commission. The advertiser must convert that visitor first. It is in the best interest for the affiliate to send the most closely-targeted traffic to the advertiser as possible to increase the chance of a conversion. The risk and loss is shared between the affiliate and the advertiser.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding targeted baselines. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.
The phrase, "Affiliates are an extended sales force for your business", which is often used to explain affiliate marketing, is not completely accurate. The primary difference between the two is that affiliate marketers provide little if any influence on a possible prospect in the conversion process once that prospect is directed to the advertiser's website. The sales team of the advertiser, however, does have the control and influence up to the point where the prospect signs the contract or completes the purchase.
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