The 5 second answer
In order to track all visitors and give a full picture of their journey all of your site needs to have CANDDi tracking in place.
The 5 sentence answer
By not having CANDDi installed throughout your site it is likely that not all visitors will be logged. A consequence of this is that some visitors may not be identified and not all information may be completely accurate. We always advise using the CANDDi tracking check (details here) to double check everything is working correctly.
If you don't achieve a perfect score, don't worry. There's advice on what this means and how to fix this on the tracking check.