Methods and apparatus to calibrate audience measurement ratings based on return path data

Methods and apparatus to calibrate media ratings based on return path data are disclosed. An apparatus includes programmable circuitry to execute machine readable instructions to: determine a subscriber calibration factor for media based on first panel tuning information obtained from first metering devices in first panelist households that subscribe to a content provider that provides media through return path data (RPD) devices; determine a nonsubscriber calibration factor for the media based on second panel tuning information obtained from second metering devices in second panelist households that do not subscribe to the content provider; and determine a final rating for the media based on an initial rating and at least one of the subscriber calibration factor or the nonsubscriber calibration factor, the initial rating based on RPD tuning information obtained from RPD devices in subscriber households that subscribe to the content provider.