Human beings are exceptionally well prepared to deal with their environment. Our senses let us capture all the important environmental influences. Our brain is trained to make sense of our perceptions.
But sometimes our senses deceive us. Surely everybody knows some examples of optical illusions. A nice collection of optical illusions is available at: http://www.michaelbach.de/ot/index.html
The following image for example shows two squares with the letters A and B that have exactly the same color (# 787 878). However our brain tries but make sense of the colors depending on light and shadows and thus will interpret the colors and so they seem to be different.(Picture under GNU license taken from weltderwunder.de.msn.com )
You see that we can not rely solely on our perception and observation to describe the reality.
We need a measurement – with the measurement observations are quantified to avoid deception (if you prepare and interpret the measurement well).
Just as in the physical world, this statement applies to websites. The measurement of the behavior of visitors and the interpretation of this measurement is commonly referred to as web analytics.
Web analytics is used to avoid deception and make decisions based on a secured knowledge of the user’s behavior and thus contribute to the success of a website.