Once the WBS is created, the work packages in it are further decomposed into activities. Activities from different work packages will have to be sequenced in order to progress and complete the project work. This creates logical relationships between the activities also known as network logic, which are of 4 types:
FS – Finish to Start – an activity must finish before the next can start; this is the most common one used.
FF – Finish to Finish – an activity must finish before the next can finish. For example level concrete can finish only when pouring concrete also finishes
SS – Start to Start – an activity must start before next can start. For example, laying of foundation pillars can start only after digging of the foundation has started
SF –Start to Finish - an activity must start before next can finish, this is the rarest and should be avoided. An example is: install of new oil pipeline must start before shutting off the old pipeline can finish
As you can see, the logic will depend on the nature of the work or industry.
The most popular method used for sequencing is the Precedence Network Diagramming method. Activities are connected in such a diagram, thus showing their sequence from the start to finish of the project. Some of them may be sequenced even in parallel or with overlap.
Activities have dependencies which are of 4 types
Mandatory (hard logic) – this is due to the nature of work or a compliance requirement
Optional or discretionary (preferred or soft logic) – this is due to best practice or desire. For example, doing the plumbing and electrical work separately even though it can be done in parallel. Removal of such dependencies usually saves time.
External – for example, waiting for a supplier to send raw materials
Internal – for example, waiting for a project team member to complete the design before it can be given to someone for review