There are various cases where cookies can be disabled: browser has cookies disabled, Matomo (Piwik) JavaScript code disabled cookies via call to disableCookies(), or if you are importing server logs in Matomo which do not contain cookie information.

When cookies are disabled, some data in Matomo (Piwik) will be less accurate. Specifically…

  • Matomo (Piwik) uses cookies to store a unique visitor ID, used to recognize visitors from previous visits. When cookies are disabled, Matomo will still be able to determine unique visitors based on IP address and other footprints, but this will be slightly less accurate.
  • Ecommerce and Goal tracking attribution: By default, Matomo (Piwik) stores the website or keyword or campaign used to find the website in a cookie, so that we can credit this website when the visitor makes a conversion (Goal conversion or Ecommerce order). If the conversion happens in the same visit, Matomo will always credit the referrer (even if cookies are disabled). However imagine the following use case: the visitor uses a newsletter link tagged with a campaign, visits 5 pages on your website but does not buy. He then comes back 2 hours later directly typing your website URL (therefore a Direct Entry visit), then buys during this second visit. When cookies are enabled, Matomo credits the conversion to the initial Campaign. When cookies are disabled, the conversion is not attributed to the original newsletter link as this data is not available.
  • Reports such as “Days since last visit”, “Visits by visit count”, “Visits to Conversion”, “Days to Conversion” will count all visits without cookie similarly to new visitors.

See also: How does Matomo (Piwik) detect unique and returning visitors?