There is widespread disagreement over how to accurately measure online traffic and visitors to websites. Content providers often report much higher internal traffic counts than those recorded by third-party measurement firms like Nielsen and comScore, who use panel-based sampling techniques. However, server-side metrics used by content providers have their own limitations. There is no consensus definition of a "unique visitor" and many alternative measurement firms have emerged but also have limitations. As a result, there is confusion in the industry over how to value digital content and online advertising. Some are advocating for new metrics like time spent to better capture audience engagement.