The document discusses how website analytics can be used to measure user interactions and the impact of small changes on online sales. It provides examples of measuring different marketing campaigns and comparing their unique visitors, time on site, goal conversions, and cost per acquisition. The document advocates testing different page designs and elements using tools like Google Website Optimizer to directly ask customers through live experiments which versions perform better rather than relying only on opinions. This allows moving from opinion-based to data-based decision making.