Relationships within hierarchies: important rule

Often the attributes linked to the dimensions in a hierarchy are closely related to each other and are often attributes of the same entity. A customer dimension is related to the region dimension that in turn is related to the country dimension and this can be a reason to make a hierarchy in order to reduce the amount of categories of the dimensions. Dimensions of higher order (those dimensions that have the lowest amount of categories) should reside higher in the hierarchy. The product-line dimension is of a higher order then the product dimension and therefore the hierarchy is as follows: Product-line à Product.

Categories of a dimension

At run time, a dimension is filled with categories. The number of categories can be an indication to:

• Present a different type of graph;
• Decide to use replacement instead of insertion of the categories of the level below;
• Split the categories automatically into smaller groups.

If the number of categories exceeds the limit of 6, a pie chart would be inappropriate (Zelazny, 1996) or the rest of the categories should taken together in a single group ‘Other...’. Moreover, pie charts cannot be used if among the categories there are positive as well as negative numbers. In that case, a bar chart is a better choice.