1. A method of determining the outcome of performing an aggregation-type operation on facts of a structured collection of facts, comprising: performing an aggregation operation on a subset of facts sampled from the full structured collection of facts, to determine an aggregation result; and based on the determined aggregation result and on an indication of characteristics of the sampled subset of facts relative to the full structured collection of facts, determining an indication of a difference between what would be the result of the aggregation-type operation on the full structured collection of facts and the actual result of the aggregation-type operation on the sampled subset of facts.