Why is it considered a good best practice to exclude NULL user_id values from reach and frequency queries?

Question

Accepted Answer

Reach and frequency cannot be accurately determined for NULL user_id values, so this group is excluded to prevent NULL user_id values from skewing the results.

Answer

Because NULL user_id values are deemed invalid traffic in post-impression server-side models.

Answer

This would be redundant; records with NULL user_id values have been removed upstream of query processing anyway.