Changing one part of a webpage can affect viewing of other unrelated parts of the page (obvious, yes, but interesting to quantify the effect). This Eyetools before / after test showed that a right-hand navigation bar received significantly increased viewing (and usage) despite the fact that it did not change at all.
This post was originally posted here, but upon seeing this analysis, I wanted to take a second try at it, which you are now reading.

The Navigation Bar	Old Page Design	New Page Design
	·
Eyetools Heatmapsâ„¢ showing group viewing trends on each web page (Eyetools Heatmap Legend)

The San Francisco Police Department redesigned their homepage (see eyetracking heatmaps below for each site). Overall, the new homepage increased reading and time-on-page, while promoting more of the department’s numerous services, both in terms of what people clicked on as well as the number of elements viewed and read.

The surprising thing was the effect the redesign had on their right Nav (which didn’t get changed at all).

The behavior on these two identical navs on two different pages was strikingly (and statistically significantly) different: the nav on the new page was clicked by 64% of our test participants as opposed to only 14% on the old website. People looked at the new site’s right nav longer, more often, and read more â€â€? despite there being no change to the design of the right navigation bar at all.

The moral of the story: A change on one part of the page can impact other, unrelated elements on the page. The right navigation bar was used completely differently on the new re-designed website because the content to the left of it changed.

Is this good or bad? Ultimately, that’s up to the client to decide as it relates to the business goals of the page. However, in this case, it was good â€â€? it enabled visitors to quickly locate the specific content they wanted.