Why is it considered a good best practice to exclude NULL user_id values from reach and frequency queries?
- 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.
- 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.
Explanation: 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** because NULL user_id values indicate that the specific user information is missing or unavailable. Since reach and frequency metrics are designed to track unique users and the frequency with which they are exposed to ads, having NULL values for user IDs makes it impossible to accurately attribute these metrics to specific individuals. Including such records would lead to inaccurate calculations, potentially distorting the results and leading to misleading conclusions. Therefore, excluding NULL user_id values ensures the integrity of the reach and frequency analysis, allowing for more accurate measurement of campaign performance.