Web users spend 69% of their time viewing the left half of the page and 30% viewing the right half. A conventional layout is thus more likely to make sites profitable.

According to research undertaken by Jakob Nielsen, each bar shows the amount of time users spent on fixations within a 100-pixel-wide stripe running down the screen, starting from the very left.

People spent more than twice as much time looking at the left side of the page as they did the right:

• Left half of screen: 69% of viewing time
• Right half of screen: 30% of viewing time

The remaining 1% of viewing time was spent to the right of the initially-visible 1,024 pixels. Such information is visible only after horizontal scrolling, and the minute amount of attention it attracts confirms the guideline to avoid horizontal scrolling

Reference

http://www.useit.com/alertbox/horizontal-attention.html