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Cradle from 3SL, the complete Model Based Systems Engineering Toolsuite, specialising in requirements management, requirements capture, model based systems engineering and for systems engineering software, support and consultancy, the logical choice: Cradle from 3SL.

January 2010 [Cradle 6.1]

Item Hierarchies

Hierarchies are very common in requirements management and systems engineering:

  • Requirement hierarchies
  • System Breakdown Structures (SBSs)
  • Cost Breakdown Structures (CBSs)
  • Work Breakdown Structures (WBSs)
  • Risk register

Cradle allows hierarchies to be constructed using drag-and-drop to create links from each parent to its children. The resulting hierarchies can be explored as trees:

and displayed as graphical Hierarchy Diagrams (HIDs):

To create a hierarchy of information, your project schema needs:

  • An item type for the items in the hierarchy
  • Appropriate characteristics for this item type to allow it to be hierarchical and to allow the hierarchy to be manipulated easily
  • Cross reference link rules to allow the hierarchy’s cross references to be created

As an example, suppose you need a System Breakdown Structure (SBS) in your project.

Login to your project as a user who has PROJECT privilege (to allow the schema to be changed), select Admin → Project Setup and select the Item Types tab in the Item Definitions group:

Enter a name for the new item type, such as SBS, and select Add to add it to the schema. You can define foreground and background colours to be used when the items appear in trees and Hierarchy Diagrams:

To set the characteristics of the SBS items to support hierarchies, select Numbering…:

By default, items are not auto-numbered, so their Number attribute will be used for the hierarchical number, such as 1.2 and 1.2.1.

However, if the structure of the hierarchy changes, then the hierarchical number will need to change. You cannot change the Number attribute as it is part of the items’ identities. So all you can do is copy the item and delete the original, which is not good as you would lose all of the change history information.

The recommended approach for items in a hierarchy whose structure may change is to set Enable Auto-Numbering and to store the hierarchical number in either the Key attribute or in a user-defined category code:

The Enable Hierarchical Options checkbox must be set to allow hierarchies of items of this type. If not set, the Cradle UIs do not allow you to perform any of the hierarchy operations, including New → Child, New → Sibling and New &arr; Hierarchy.

To ensure that cross references can be created in the hierarchy, you will need to add a link rule to allow cross references from SBS items to SBS items.

In Project Setup, select the Link Rules tab in the Cross Reference Parameters group:

There are no link rules in this schema, so a link rule to allow hierarchical links for SBS items is not essential as there is no link rule to prevent such cross references. In general there will be a default link rule that prevents all cross reference operations:

which is preceded by link rules that specify what is allowed. The link rule to allow cross references between the SBS items would be defined:

This creates a small set of link rules for this example:

This completes the changes to the project schema to allow a hierarchy of SBS items. In a real project, further characteristics would be defined, particularly the frame and category code attributes that will be needed by the SBS items and the manner of recording changes to them.