There are various cases where cookies can be disabled: browser has cookies disabled, Piwik JS code contains call to disableCookies(), or if you are importing server logs in Piwik which do not contain cookie information.
When cookies are disabled, some data in Piwik will be less accurate. Specifically…
- Ecommerce and Goal tracking attribution: By default, 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, Piwik 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, Piwik 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.