In those cases, the data you gather for comparative purposes may be woefully incomplete. If the barrier is at your own site, you may be able to convince your Webmaster to change the basic design of the pages or to create a duplicate set of plain, indexable pages. If the barrier is at a partner's site, you may want to encourage the Webmaster to allow AltaVista Search to index the site so that people will be aware that it exists and that it contains valuable business information. If it's at a competitor's site, you should simply give up on the idea of comparative analysis based on AltaVista Search and be content with the knowledge that your competitors are missing important traffic. The indexing barriers you'll encounter include registration, frames, databases, dynamic pages, and text in Acrobat or PostScript.
Some sites require users to complete a form before they can advance to the real content.
In some cases, the purpose is simply to capture information about users because access to the site is free. Web crawlers, like Scooter, are dumb robots. Because they can't fill out forms or supply passwords, they can't access sites containing indexing barriers. Normally, such sites will allow crawlers to index their home pages but none of the remaining content. But if all the pages are password-protected, you can't index them.