At the 2009 tekom conference in Weisbaden, Germany, Scott DeLoach of ClickStart Inc. gave an excellent overview of what research has found on the usability of user assistance. Research has found that

• Users prefer Arial over Verdana and Tahoma. The two latter fonts were developed for onscreen reading. People tend not to do extensive reading on-screen.
  TNR scored the same as Verdana when reading short texts.
  
  Many users have problems reading on-screen text that is below 10 point. Older users prefer text to be 12/14 point.
• Users don’t like scrolling. They prefer to read a screen than scroll down it to find the information.
• Users want quick answers. They focus on minimizing effort rather than maximizing learning. “Easy to find” ranked higher than “Correct information”. They are focused on completing a task.
  Users scan information; they don’t read long sentences. Limit sentences to 20 words.
• Novices use introductions to remember information and build knowledge. However, experts are frustrated by overviews. Experts scan for procedures, notes, and tips.
  Consequently don’t put technical information in overviews.
  Users learn from examples so FAQs are very popular.

Linking guidelines

• Users spend up to 80% of their time in the first screen. Many prefer to move to another screen than to scroll down. Therefore don’t put links at the bottom of a screen.
  Preferably place links in the right margin of a screen.
• Use text links rather than image links. This is mainly because they change color when clicked. Text is also easier to customize that an image.
• Use links that are descriptive. Don’t simply say “Help”. Instead, for example, say “How do I …”, “Quick tips” ….

You can find more information on this topic at http://www.clickstart.net/?page_id=8