By week three of the March book (remember that Nielsen "weeks" are not calendar weeks and often have pieces of prior or next months). 12+ AQH listening in Chicago was off by about 13%. 25-54 was off by closer to 18%.
Many people at home are not listening to broadcast AM and FM, and may not always be hearing stations on a medium that is encoded.
Remember, subscribers get weekly data, covering the about three weeks prior.