Methods and apparatus to generate audience measurement data from population sample data having incomplete demographic classifications are disclosed. An example method includes detecting that a number of impressions a time segment of a video occurred based on messages; detecting that no impressions for the time segment of the video were detected by the database proprietor; determining respective percentages of exposures of the video that are attributable to corresponding ones of multiple demographic groups, determining the respective percentages being based on the first demographic information provided by the database proprietor for exposures of other time segments of the video; attributing respective portions of the number of impressions to the demographic groups based on the respective percentages to determine imputed numbers of impressions; generating adjusted numbers of impressions based on the imputed numbers of impressions; and determining ratings information for the video using the adjusted numbers of impressions.