Sequence Activities – Tools To Establish Logical Relationships.

How To Sequence Project Activities
Steps to Sequence Project Activities

While developing the project schedule it is imperative that all project activities that relate to project success are identified. These identified activities are then analysed for relationships that exists among them so that a well structured and a logically correct network of project activities can be established.  A logically correct schedule network can help the project team in concentrating on sequence of critical activities that needs to be performed in order to deliver the project within budget and schedule. No matter which schedule development technique is used activities have to be sequenced based on logical relationship that exists between them.

This post intends to look into various tools and techniques that are available to sequence various project activities. Since Precedence Diagramming Method (PDM) is predominantly used to construct schedule network diagram in which activities are indicated on nodes and are graphically linked to each other by logical relationships to show the sequence in which the activities will be performed. Activity-on-Node (AON) is one method of representing a PDM.

(1). Logical Relationships:

Four types of dependencies are used in establishing logical relationships. However, before knowing what these dependencies are it is important to understand what are predecessor and successor activities.

Predecessor Activity:

It is an activity that comes before a dependent activity in a project schedule.

Successor Activity:

It is an activity that comes after a dependent activity in a project schedule.

Predecessor_Successor_Activity
Predecessor & Successor Activity

In the figure above, activity P is predecessor activity of A and activity S is successor activity of A. However, activity A is also the successor activity of P and predecessor activity for S

The four dependencies used in establishing logical relationships are as follows

(a). Finish-to-Start (FS)

It is a logical relationship in which a successor activity (Activity B) cannot start until a predecessor activity (Activity A) has finished. It is the most commonly used dependency in scheduling software to establish logical relationship.

Finish-to-Start
Finish-to-Start

(b). Start-to-Start (SS)

It is a logical relationship in which a successor activity (Activity B) cannot start until a predecessor activity (Activity A) has started.

Start-to-Start
Start-to-Start

(c). Start-to-Finish (SF)

It is a logical relationship in which a successor activity (Activity B) cannot finish until a predecessor activity (Activity A) has started.

Start-to-Finish
Start to Finish

(d). Finish-to-Finish (FF)

It is a logical relationship in which a successor activity (Activity B) cannot finish until a predecessor activity (Activity A) has finished.

Finish-to-Finish
Finish-to-Finish

(2). Dependency Determination:

Project activities may also have following four attributes which help in establishing logical relationship.

  1. Mandatory Dependencies
  2. Discretionary Dependencies
  3. External Dependencies
  4. Internal Dependencies

Any two dependencies may be present at the same time in the following manner

  • Mandatory External Dependencies
  • Mandatory Internal Dependencies
  • Discretionary Internal Dependencies
  • Discretionary External Dependencies

(a). Mandatory Dependencies

  • Also known as Hard Logic or Hard dependencies.
  • Legally or contractually required or inherit in the nature of work and involve physical limitations.
  • Technical dependency may not be mandatory and are not schedule constraints.
  • Project team determines which dependencies are mandatory.

(b). Discretionary Dependencies

  • Also known as preferred logic, preferential logic, or soft logic.
  • Established based on best practices of an application area or when a specific sequence is desired.
  • Improper use may result in arbitrary total float and during fast tracking these should be reviewed for modification or considered for removal.
  • Project team determines which dependencies are discretionary.

(c). External Dependencies

  • Involve relationship between project activities and non-project activities.
  • These dependencies are usually outside the project team’s control.
  • The project management team determines which dependencies are external.

(d). Internal Dependencies

  • Involve precedence relationship between project activities and are generally inside project team’s control.
  • The project management team determines which dependencies are external.

(3). Leads & Lags:

Lead:

Lead is amount of time the successor activity (B) can be advanced with respect to predecessor activity (A). Lead is represented as a negative value of lag. For example FS-10 days

Lag:

Lag is amount of time a successor activity (B) will be delayed with respect to predecessor activity (A). For example it is represented as SS+10 days.

 

Leads_Lags
Leads and Lags

Conclusion

Establishing logical relationship among various project activity is an important step in the schedule development process. Wrong relationships result in improper identification of critical path and puts the schedule control and resource allocation at risk.

To understand how to analyse a project schedule you may read

For steps involved in critical path analysis you may read

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