A method of defining aggregate levels to be used in aggregation in a data store having one or more dimensions. Levels are defined corresponding to attributes in the dimension, so that data can be aggregated into aggregates corresponding to values of those attributes. The invention provides for the definition of sub-levels which act as levels but define which detail entries in the associated dimension will contribute to the sub-level. The invention also provides for the definition of level groups. A level group can replace a level in a level cross-product and such a cross product is then expanded before aggregation into a set of cross products, each containing one of the level group entries.