1. A method for estimating a probability of error of a value of a particular attribute in an aggregated database, the aggregated database being compiled from three or more source databases containing values for at least some of the same attributes as each other, the method comprising the steps of: estimating a probability of error for each source database, the probability of error for a source database representing the chance that values of attributes in the database are incorrect; and estimating the probability of error of the value of the particular attribute in the aggregated database from the estimated probabilities of error for the source databases, wherein the step of estimating the probabilities of error for the source databases comprises: determining numbers of conflicts between respective pairs of the source databases by, for each pair, comparing values of attributes in the pair of the source databases, the attributes of the pair whose values are compared being the same; and deriving the probability of error for the source databases from the numbers of conflicts.