Methods, apparatus, systems, and articles of manufacture are disclosed to generate synthetic respondent level data. An example apparatus includes: a comparator to compare a target rating to a computed rating to determine a comparison result, the computed rating determined based on a seed panel, the seed panel including monitored panelists associated with return path data; a seed panelist data adjuster to adjust the seed panel based on the comparison result to reduce an error between the target rating and the computed rating; and a household data adjuster to add tuning without viewing data to households of the adjusted seed panel, the tuning without viewing data for a first one of the households to represent monitored data corresponding to a media presentation device of the first one of the households being on while a media output device in communication with the media presentation device is off.