Example methods, apparatus, systems and articles of manufacture to implement under-addressable advertisement measurement are disclosed. Example apparatus disclosed herein determine a tuned under-addressable advertisement duration and a total tuned advertisement duration for a first television broadcast at a first monitored site, the tuned under-addressable advertisement duration and the total tuned advertisement duration determined based on (i) return-path tuning data associated with the first monitored site, (ii) linear advertisement log data associated with a broadcast area including the first monitored site, and (iii) addressable advertisement log data associated with the first monitored site. Disclosed example apparatus also determine an under-addressable advertisement factor for the first television broadcast based on the tuned under-addressable advertisement duration and the total tuned advertisement duration, and adjust an input advertisement rating for the first television broadcast based on the under-addressable advertisement factor to determine an under-addressable advertisement rating for the first television broadcast.