Products
VDM
Issue
Multiple visitor records have the same cookie value.
Cause
VDM starts a session by using the visitor cookie to identify the visitor. If a visitor record exists, it updates the appropriate data points for the visitor. If not, it creates a new visitor record.
VDM reads through the session, typically using WT.vt_sid to maintain a constant session ID. If VDM sees a different cookie value while still using the same session ID, it looks for the visitor record tied to the new cookie value. If it finds a visitor record, it updates it with the first cookie value it saw in the session. This technique allows queries to tie together all visitor records where a visitor may have had several cookie values throughout their lifetime.
Resolution
The most common cause of cookies changing mid-visit is tagging. Check your site to make sure all tag version are consistent and that first-party cookie values (WT.co_f) are sent consistently across your entire site. A common issue is the ?fpcdom? setting in tags are set incorrectly. Be sure it is set consistently across the site and matches any previous tags you may have had on your site.