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

• Because NULL user_id values are deemed invalid traffic in post-impression server-side models.
• This would be redundant; records with NULL user_id values have been removed upstream of query processing anyway.
• 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.

The correct answer is:

• 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.