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

Excluding NULL userid values from reach and frequency queries is a best practice because these records cannot contribute to accurate measurement. Reach and frequency metrics rely on identifying unique users to count how many distinct individuals were exposed to an ad (reach) and how many times on average each unique individual was exposed (frequency). A NULL userid means the system cannot identify a specific user, making it impossible to determine if that impression or click belongs to a new user or a returning one. Including them would skew the results, leading to an overestimation of reach or an inaccurate calculation of frequency. The other options are incorrect because NULL userid values are not necessarily invalid traffic, nor are they automatically removed upstream; they simply lack the necessary identifier for these specific metrics.