Methods and apparatus to generate corrected online audience measurement data

Methods and apparatus to generate corrected online audience measurement data are disclosed. An example apparatus includes an impressions collector to receive a first audience count indicative of first impressions attributed to a first media category for a demographic group, a second audience count indicative of second impressions attributed to a second media category accessed by the demographic group, and a third audience count indicative of third impressions attributed to the first media category and the second media category accessed by the demographic group, the first audience count including a duplicated audience count; a factor generator to calculate a deduplication factor using the first audience count, the second audience count, and the third audience count; an audience calculator to determine a corrected audience count of the first media category to correct the duplicated audience count; and a ratings data generator to generate ratings information for the first media category.