1. An apparatus comprising: logic instructions operable to: generate a first estimate of the number of unique values of an attribute in a data sample using a first estimator; generate a second estimate of the number of unique values of the attribute in the data sample using a second estimator; generate a weighted first estimate based on a weighting factor and the first estimate; generate a weighted second estimate based on the weighting factor and the second estimate; and combine the weighted first and second estimates to form a final estimate.