1차 출처 : http://www.talkqueen.com/Google-Analytics-000000-Avg-Time-on-Site-mystry-solved-q117008

2차 출처 : https://www.en.adwords-community.com/t5/Measure-results/Why-Avg-Page-Visit-Duration-shows-00-00-00/td-p/18093

If you have a blog or web site and use Google Analytics as your analytics package, you may have come across a disappointing statistic: visits of no length, or 00:00:00. several keywords I’ve registered average visit lengths of no length whatsoever (00:00:00). That doesn’t sound good. Keywords in rows are the 00:00:00 visits. They also have something else in common. Each of these visits is of only one web page (the value in the Pages/Visit column is 1.0 for each of these keywords). Single page visits, called Bounces in most analytics packages are handled different ways by different packages. Before moving forward, let’s be clear about what a Bounce is. A Bounce, or Single Page View Visit is a visit in which the user arrives at a web page, and then leaves the web site altogether. If the user even refreshes the same page, it is not a Bounce. A bounce is someone arriving at your site, then doing one of several things such as clicking the back button, typing a new destination into the address bar of their browser, closing their browser program altogether (you get the point - basically anything but continuing to view pages on your web site). OK. So all of our 00:00:00 visits were also Bounce visits. That still seems quite surprising. None of the visits registered even a second on the site? Well, the visits may have been a second, ten seconds, or several minutes. We just don’t know, and we don’t know due to the way Google Analytics calculates Time on Site (or Time on Page). To calculate how long a user is on a page, Google needs two data points. It needs the time when a page was loaded (which in our case it has) and the time when a subsequent page was requested from the site (which it does not have). So, in lieu of that last piece of data, Google classifies these Bounce visits as 00:00:00 in length. It would also stand to reason that every visit to a web site is registered as shorter than it actually is as the amount of time spent on the last page of the site prior to exiting can’t be calculated. On a side note… If you are reviewing metrics on individual pages, Google does not include Bounce visits into the equation when it calculates the average time spent on that page. For the average time spent on site, however, it does. hope this relieves you all involved in similar situation.

요약

1 페이지뷰 방문을 bounce라 하는데 이 때에는 체류 시간을 알 수 없다. 체류 시간은 방문 시작 시간, 다음 페이지 방문 시간으로 알 수 있는데 방문 후 바로 나가버리면 체류 시간을 계산할 수 없다. 첫 페이지에서 새로 고침을 하면 체류 시간을 알 수 있으나 뒤로 가기 버튼 클릭, 다른 웹사이트로 이동, 닫기 버튼 클릭 등의 이벤트가 발생하면 analytics 툴에선 이를 인지하지 못하고 체류 시간을 0초로 계산해버린다.