Methods, apparatus, systems, and articles of manufacture are disclosed to generate synthetic respondent level data. Example apparatus disclosed herein include a synthetic panel generator to (i) generate a synthetic panel corresponding to a duration of time, the synthetic panel to be generated based on (a) a seed panel and (b) duration constraints based on return path data reported by a plurality of media devices, the synthetic panel representative of audiences of media presented by the plurality of media devices during the duration of time, and (ii) generate synthetic respondent level data based on viewing data of synthetic panelists in the synthetic panel. Disclosed example apparatus also include an output file generator to generate an output file based on the synthetic respondent level data.