Jakob Nielsen's new book, Prioritizing Web Usability, is a worthy companion to the previous two. Now it's a trilogy:

1. Designing Web Usability: The Practice of Simplicity (2000)
2. Homepage Usability: 50 Websites Deconstructed (2002)
3. Prioritizing Web Usability (2006)

You can tell Jakob and his co-authors are growing ever more skilled at the practice of simplicity; this book is the first in the series to drop the colon and subtitle.

The very existence of the new, updated book hints that usability guidelines evolve over time. In one of the first chapters, Nielsen makes this explicit by revisiting earlier web usability issues to see how much they've improved in seven years. Each usability issue is rated from zero to three skulls to indicate how severe the problem is today:

1. Usability issues that are still major problems today

	Links that don't change color when visited
	Breaking the back button
	Opening new browser windows
	Pop-up windows
	Design elements that look like advertisements
	Violating web-wide conventions
	Vaporous content and empty hype
	Dense content and unscannable text

2. Usability issues that are less important due to improvements in technology

	Slow download times
	Frames
	Adobe Flash content
	Low-relevancy search listings
	Multimedia and long videos
	Frozen layouts
	Cross-platform incompatibility

3. Usability issues that are less important because users have adapted to the web

	Uncertain clickability
	Scrolling
	Registration
	Complex URLs
	Pull-down and cascading menus

4. Usability issues that are less important because designers have learned restraint

	Plug-ins and bleeding edge technology
	3D user interfaces
	Bloated design
	Splash pages
	Moving graphics and scrolling text
	Custom GUI widgets
	Not disclosing who's behind information
	Made-up words
	Outdated content
	Inconsistency within a web site
	Premature requests for personal information
	Multiple sites

When comparing the severity of these 34 usability issues with their historical severity in 2000, Nielsen notes that most of the progress can be attributed to designers learning restraint:

Relying on user education or technology fixes to address usability issues means you'll be waiting a long time. Most of the immediate benefits are realized by designers who learn to follow usability guidelines. But designers are fallible, too, so there's no guarantee these problems won't crop up again later, or in slightly different forms.

The data presented in Prioritizing Web Usability shows that usability guidelines do evolve over time, but slowly. It also illustrates how the core principles of usability are timeless:

From 1984 to 1986, the U.S. Air Force compiled existing usability knowledge into a single, well-organized set of guidelines for its user interface designers called Guidelines for Desigining User Interface Software, ESD-TR-86-278 (also available as a pdf). Jakob Nielsen was one of several people who advised the undertaking. The project identified 944 guidelines, most of them related to military command and control systems built in the 1970s and early 1980s, which used mainframe technology.

You might think that these old findings would be irrelevant to today's designers. If so, you'd be wrong. As an experiment, we retested 60 of the 1986 guidelines in 2005. Of these, 54 continue to be valid today. Of the total 944 guidelines, we deduced that 10 percent are no longer valid and 20 percent are irrelevant because they relate to rarely used interface technologies. But nearly 70 percent of the orginal guidelines continue to be both correct and relevant 20 years later.

This is one of the reasons I urge software developers to study and understand the principles of usability. It's one of the precious few bodies of knowledge in a developer's toolkit that will still be useful twenty years from now.