A favourite method is to ensure that a website establishes links on busy or related sites; this, of course, can be expensive. Website design is also important in terms of initial attractiveness, navigation aids around the site, and so on. Loyalty programmes modeled on airlines’ frequent-flier offers are also beginning to appear on the web. Database marketing can also be used to develop relationship with customers. This is the key to Amazon’s success. One of the biggest problems facing e-commerce relates to the concerns which customers have about credit card security. To alleviate these fears stores have adopted security measures such as encrypting transactions (use of codes such as PIN numbers) and using ‘firewalls’ (blocks) to protect customer data stored on their computers. These measures obviously add to costs. In mid-1999 there were estimated to be 150 million users worldwide with over 8 million in the UK.