Technologies relating to determining online advertisement statistics are described. In general, one aspect of the subject matter described in this specification can be embodied in a method that includes receiving a prediction value for a click-through rate for an online advertisement, the prediction value indicating a probability that the online advertisement will be accessed when displayed in an online medium, the prediction value based on a ratio of a number of times the advertisement has been accessed by users to a number of times the advertisement has been displayed to users, identifying at least one condition that is present when the online advertisement is to be displayed, the condition affecting accuracy of the prediction value, and generating a corrected prediction value based on at least one stored shift value associated with the at least one condition.