Calculating ES, EF, LS, LF and Float using Precedence Diagramming Method (PDM)

Note:

These all  articles are dedicated to my teachers, my seniors, colleagues, internet bloggers, online technical material, reference books from where I learned all these. There are chances that the material presented here is duplicated somewhere on the web. If anything is replicated anywhere, I sincerely give proper credits to the contributor.

Here we are going to do almost everything which is possible with a network diagram. This exercise will help you strengthen your understanding of PDM (Precedence Diagramming Method), Early and Late Dates and Float.

Not going into too much theory let’s start with the following table which shows the activites and their dependencies.

Most importantly I am going to relate each step with the processes of Time Management, which I described in my earlier blog:

From above dependencies, we make Network Diagram as below:

 

Wasn’t it simple? That is the advantage of doing it step by step.

Let’s write down the duration on the above diagram.

So you noticed that it is Activity on Node (AON). We have put the Activity (A, B, C etc.) in those squares called nodes.

In the Example:

Path A->B->E is 14 days Duration

Path A->C->D->E is 10 days Duration.

So longest path is 14 days and that is Project Duration. It is called Critical Path.

• Longest Path in the Network diagram decide duration of the project. It is called Critical Path.
• Path which are shorter then Critical Path are called Non Critical Path.

Let’s make a detailed Network Diagram now.

Please note: ES – Early Start, LS – Late Start

                   EF - Early Finish, LF – Late Finish

 

1. Early Start (ES) and Early Finish (EF) are written at Top Corners. We will enter these values in Upper Left and Upper Right Cell Later.
2. Late Start (LS) and Later Finish (LF) are written at Bottom Corners. We will enter these values in Lower Left and Lower Right Cell Later.
3. We have entered the duration of each activity in Upper Middle Cell. Name of the Activity is put in Center Cell.
4. Now let’s say the Start date is August 01 i.e. 01/08. So this is the Early Start. So for the activity A Early Finish (EF) will be 02/08 as the duration is 2 days. Moving Forward (Forward Pass) towards the end of the Project, Lets enter all early dates.

1. So now we understand what an Early Start (ES) is. For example Activity “C” it can be started as early on 03/08 because Activity “A” is finished on 02/08.
2. Forward Pass: Finding the Early day by travelling from First Activity to End of Project is called Forward Pass.

Now we will look for Late Dates. i.e. Late Start and Late Finish.

Late Finish for Last Activity will be same as Early Finsh (EF) as Project cannot be delayed, so the End date (Whether EF or LF) will be same.

So for Activity “E”, Late Finish (LF) will be also 14/08.

Now we will move backwards on Critical Path (A->B->E). We will move E->B->A

1. Now the LF for “E” is 14/08 and duration is 2 days, so LS will be 12/08
2. So moving backward we will get Late Dates for E, B and A.

Note that for Critical Path (Longest Duration Path A->B->E, Late Finish Dates are same as Early Finish

Dates. Similarly Late Start Dates are same as Early Start Dates.

Now we will calculate the Late Dates for Activities “C” and “D” which are on Non-Critical Paths.

In the Network Diagram “D” is Predecessor of “E” i.e. “E” can be started only when “D” is finished. Since “E” can be lately started by 13/08, “D” can be lately finished by 12/08. So the Late Finish Date (LF) for “D” is 12/08.

Since the Duration of “D” is 3 days, so the Late Start (LS) will be 10/08. Now this means we can delay Activity “D” by 4 days without affecting the Project Schedule because the Project is still being finished at 14/08.

This duration of “4” days is called “FLOAT” or “SLACK”

Lower Center Column is for filling the Float. Also note that on the Critical Path, If we calculate the Float it is Zero.

Similarly we can calculate the Late Finish and Late Start of C.

Now let’s fill these dates and Floats in Network Diagram.

Forward Pass is done from First Activity to Last Activity and Backward Pass is done from Last Activity to First Activity.

Float or Slack is also called Scheduling Flexibility

In above example, for Activity “E”, Early Finish is 14th. Late Finish also has to be 14th only as we have not to delay the project. So, for Activity “E” there is no Scheduling Flexibility. Therefore we can’t help.

Since Activity E is the last activity of the project any delay in Its competition will delay the Project. Early finish is 14th late finish is also has to be 14th only we do not have any schedule flexibility therefore we cannot help.

 

• In Critical Path Activities there is no difference between Late Dates and Early Dates or there is no float or there is not schedule flexibility.
• In Non-Critical Path Activities: There is difference between Late Dates and Early Dates or there is float of Schedule flexibility.
• Float should not be confused as duration of the activity or time buffer. Float comes because of sequencing. You change the sequence Float will change. But duration of the activity and time buffer will not change.
• Checking float variance during execution is an important technique of controlling schedule.

 

Float of NCP Activity

Length of CP – Length of NCP on which activity is found.

In above Example: LCP (14) – LNCP (10) = Float (4)

Float of a NCP activity shared between more than one NCP, Let say

A, B, E = 14

A, C, D, E = 10

A, C, F, E = 10

A, C, G, E = 8

Here is Activity “C” is shared on three non-critical paths.

Length of CP – Length of Biggest NCP (On which activity is shared)

14 - 12 = 2 (Float for Activity “C”)