Page "Estimator" Paragraph 2

Statistical theory is concerned with the properties of estimators ; that is, with defining properties that can be used to compare different estimators ( different rules for creating estimates ) for the same quantity, based on the same data.

Such properties can be used to determine the best rules to use under given circumstances.

However, in robust statistics, statistical theory goes on to consider the balance between having good properties, if tightly defined assumptions hold, and having less good properties that hold under wider conditions.