Google only uses a small sample of data to extrapolate what they believe is happening with a site. Although this is not a problem with small low traffic sites, it is very inaccurate for large traffic popular sites. This results in statistics such as the number of users and page views being underreported by Google up-to 35%.
(i) Keyword data is hidden for those using Firefox, Chrome Omni box, and those logged into any Google product (60% of keyword data is not provided).
(ii) Referral traffic refers to visitors who came from other websites. But in practice, it often includes traffic from social media, email and in some cases, traffic from one page on your site to another. So, it is impossible to know with accuracy where users are coming from.
(iii) Direct traffic is defined as “All traffic for which a referrer wasn’t specified”, which is a catchall and produces weird results.
Google needs cookies to read data. If the visitor’s computer rejects cookies, it can’t be included. Also, if visitors clear their cookies, they will be recorded as a new user.
This results in low numbers for those accessing the website from shared computers, such as at offices, libraries, universities, etc.
If someone accesses website from a desktop and a cell phone, it will record as two visitors.
Google does not have the astronomical computing power it would take to track all activity on the Internet.
* Source: There are thousands of articles explaining the 24 most troublesome shortcomings of Google Analytics. We simplified the top 7.
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