1. A method for estimating ad inventory in a video streaming system, by at least one computer, the method comprising: determining a probability distribution of samples taken from electronic data records of ad impressions in an interactive online video streaming service, among a population comprising each different combination of targeted attributes recorded for each of the samples, using a computer analyzing the electronic data records; populating an electronic data structure in a computer memory relating the each different combination of the targeted attributes to a forecasted number of available impressions in a defined time period, based on the probability distribution and a forecasted total number of available impressions in the time period; and estimating an inventory number of impressions in a program collection in the defined time period available for targeting ads to be included in streamed ones of the program collection providing ad impressions having a defined set of targeted attributes, based on the electronic data structure.