 <p>Respective to the different programs they are both correct. The difference comes in in <strong>HOW</strong> they calculate what a unique visitor is. No two stats aggregators work the same.</p> <p>Google Analytics <a href="http://support.google.com/googleanalytics/bin/answer.py?hl=en&amp;answer=113734">What's the difference between the 'Absolute Unique Visitor' report and the 'New vs. Returning' report?</a>:</p> <blockquote> <p><strong>Absolute Unique Visitors</strong></p> <p>In this report, the question asked is: 'has this visitor visited the website prior to the active (selected) date range?' The answer is a simple yes or no. If the answer is 'yes,' the visitor is categorized under 'Prior Visitors' in our calculations; if it is no, the visitor is categorized under 'First Time Visitors.' Therefore, in your report, visitors who have returned are still only counted once.</p> </blockquote> <p>Piwik <a href="http://piwik.org/faq/general/#faq_43">FAQs</a>:</p> <blockquote> <p><strong>How is a 'unique visitor' counted in Piwik?</strong></p> <p>Unique Visitors is the number of visitors coming to your website; Unique Visitors are determined using first party cookies. If the visitor doesn't accept cookie (disabled, blocked or deleted cookies), a simple heuristic is used to try to match the visitor to a previous visitor with the same features (IP, resolution, browser, plugins, OS, ...). Note that by default, Unique Visitors are available for days, weeks and months periods, but Unique Visitors is not processed for the "Year" period for performance reasons. See how to enable Unique Visitors for all date ranges.</p> </blockquote> <p>They both use cookies to determine uniques, but both go about it calculating them in different ways. It's apples and oranges when comparing stats packages side by side.</p> <p>Examine the rest of the stats beyond unique visitors. If there is a wide margin across the board, take a close look at the implementation of both.</p> <p>If all is well with both implementations, then pick one and go with it for the stats. Overall <strong>trends</strong> is what you are looking for. Are the stats you want to go up going up? Are the stats you want to go down going down?</p>