A badly designed intranet can cost a 10,000-employee company $15m a year in lost productivity, according to a new study from web usability guru Jakob Nielsen.

Poor search facilities, lack of consistent navigation and using bulky PDF files instead of navigational web pages are the biggest traps companies fall into and the major causes of reduced usability, the study found.

Staff in 14 companies were asked to perform a number of common tasks, including finding information about a co-worker and details of a specific employee benefit using the corporate intranet. Those completing the same tasks on a poorly designed intranet took up to seven times as long to complete them.

Jakob Nielsen, principal and co-founder of Nielsen Norman Group, told vnunet.com that companies should be spending at least 10 per cent of their web and intranet budgets on analysing usability, and the other 90 per cent on putting that into action.

"In general if you spend 90 per cent of your budget on doing it, the payback is immense. The return on investment is in the order of 1,000 per cent for the lifespan of the site," Nielsen said.

But the relative immaturity of the web design discipline meant that many companies were still falling into common traps which, rather than attract users to their sites, made navigation extremely difficult.

"Interaction design is not something that's been studied very much. A lot of companies still use guesswork or rely on market research methods that don't work for usability - for example, showing people screen-grabs and asking if they like them. They're not going to buy the website, they're going to use it," Nielsen said.

Failing to provide answers to common site visitor questions, especially prices, and confusing users with new looks for common features such as scroll bars are common gripes, according to Nielsen.

"The worst thing about the web is its opaqueness. There are a lot of mistakes because people are trying to be too clever. One company had a feature called TrackIT for technical support, but users thought it was to track packages. It confused them," he said.

"People should spend their creative forces trying to make things work well. It's not trivial to think up services and structure them in a way that works well," Nielsen added. "The average person is mystified, intimidated and bewildered by computers."

Matthew Berk, senior analyst at Jupiter Research, said considering the user experience was important, but so was ensuring sites added value to the site owner.

Jupiter predicts that over the next five years website investments will be increasingly anchored to specific business goals, and that annual spending on site analytics will reach $1bn by 2006.

"The web is eight years old commercially - but site design is completely disconnected to how people use websites. Great design is often the enemy of usability and a red herring for websites," Berk said.