Methods Necessary To Solve These Problems

Think of this section as your notebook, below are listed all different definitions necessary to solve Network Diagram questions

Forward Pass

Let's assume you have a network diagram. To perform a forward pass we begin at the left. I usually work in columns.

Step 1.  The early start of the first activity is 1.  Write 1 on the diagram in the top left corner of Activity A.

Step 2.  The early finish is equal to the early start plus the duration minus 1.  In this example for Activity A the Early finish = 1 + 5 – 1 = 5.  The early finish goes in the top right corner of the activity.

Step 3.  The early start of the next activity (the successor activity) is 1 plus the early finish of the current activity. For Activity B the early start is 5 + 1 = 6.  The same is true for Activity C.

(You need to go back to Step 2 to determine the early finish of Activity B and Activity C.)

Step 4.  Going forward if more than one path converges into a single activity the early start of that activity is 1 plus the largest early finish found on any of the converging paths.  When determining the early start of Activity E will look at the two predecessors.  Activity B has a larger early finish than Activity C.  Therefore to determine the early start of Activity E will look at the early finish of Activity B only.  Early start of Activity E = Early finish of Activity B (14 days) plus 1 day = 15.

Step 5.  Continue until you have completed the entire forward pass.

Backwards Pass

To perform a backward pass begin at the right and move to the left in columns.

Step 1.  The late finish of the last activity (Activity E) is the same as the early finish of the last activity.  Just copy the early finish of the last activity and write it in the late finish position- the bottom right of the activity.

Step 2. The late start of an activity is equal to the late finish minus the duration plus 1. For Activity E the late start = 15 -1 +1 = 15.

Step 3.  The late finish of any immediately preceding activity is the late start of the current activity minus 1. For Activity B the late finish = 15 (which is the late start of Activity E) – 1 = 14.

Step 4.  Going backwards if an activity has two successors we determine its late finish by taking the smallest late start of the successors and subtracting one.  For Activity A there are 2 successors (Activity B and C).  Activity B has the smallest late start.  The late finish of Activity A = 6 (the late start of Activity B) – 1 = 5.

Step 5.  Continue until you have completed the entire backward pass.

Forward Passes with Complex Relationships

There are four types of complex relationships when dealing with Network Diagrams, these four are described below and I have also included a diagram showing showing how a Forward Pass would work for each relationship.

Finish to Start (FS) relationships

The most common type of relationship.  All relationships are assumed to be finish to start unless otherwise stated.  Activity A must finish before Activity B may start.

Example: we finish writing code before we start testing code. 

Finish to Finish (FF) relationships

Activity A must finish before Activity B may finish. Example:  I must finish cooking the meal before I may finish serving the meal on the table.

You may not like where we drew the line between A and B for a finish to finish relationship.  On the exam we expect the line to be drawn and labeled like this figure. 

Start to Start (SS) relationships

Activity A must start before activity B may start.  I must start washing my dishes before I may start drying my dishes.

You may not like where we drew the line between A and B for a start to start relationship.  On the exam we expect the line to be drawn and labeled like this figure. 

Start to Finish (SF) relationships

Activity G must start before Activity H may finish.  This is the least common of all relationships and many of us will not use the relationship in any of our network diagrams.   Example:  the night security guard must start his shift before the day security guard may finish his shift. We don’t need to worry about the forward and backward pass for this relationship on the exam so we will skip over it here as well. 

Leads and Lags

Leads and Lags are one way that a standard Network Diagram can become more complicated.

No Leads or Lags

First we will look at a relationship without any leads or lags, as you can see this is what we have already been studying.

Leads

A lead is a modification in a logical relationship that allows the acceleration of a successor activity. In the following diagram Activity D is allowed to accelerate 2 days. In other words, Activity D may start 2 days before Activity C finishes.

Lags

A lag is a modification in a logical relationship that forces the delay of a successor activity.  In the following diagram Activity F cannot start until 2 days after Activity E has finished.

The Arrow Diagramming Method (ADM) produces Activity on Arrow (AoA) diagrams 

Activity on arrow (AOA) diagrams allow for only finish to start relationships.

 In these diagrams the arrow represents:

• The Activity
• The Precedence

These diagrams may also use dummy activities.  Dummy activities ensure that all logical dependencies are represented accurately.  Dummy activities have zero duration.

The circles (often called nodes) represent events that require no time or resources.

The nodes may have letters or numbers in the node.  If there are numbers in the node like our example below the numbers do not represent durations.  The numbers are names for the node.  Example: Node 1. 

Primary rule:  All activities that enter a node must be completed before any activity leaving the node may be started.

In the diagram below Activity A must finish before Activity B may start.

Activity A and Activity D must finish before Activity C may start.  The dummy activity tells us that Activity A must finish before Activity C may start.

Graphical Evaluation Review Technique (GERT) is used to produce GERT diagrams

GERT is a network analysis technique that allows for probabilistic treatment of the network logic and estimation of activity durations.

GERT is not common because of the complexity of GERT. 

GERT allows for conditional statements and loops between activities.  Monte Carlo simulation is often used to model GERT.

On the PMP® Exam GERT is more likely to be a wrong answer than a right answer.  Let’s look at the diagram below.  The diagram looks like a flow chart.  Imagine this flow chart with the feedback loop being part of your schedule. Can you see how the use of GERT may be very complicated in a schedule?