CPM (Critical Path Method) uses the following times for an activity.
• (EST)Early start Time is the earliest time the activity can begin.
• (LST)Late start Time is the latest time the activity can begin and still allow the project to be completed on time.
• (EFT) Early finish Time is the earliest time the activity can end.
• (LFT) Late finish Time is the latest time the activity can end and still allow the project to be completed on time.
We will use the same example of the computer institute described in the previous
example for calculating and understanding EST and EFT.
Figure: - Start and End
According to CPM calculation the start date should be minimum 1-Jan-2009 and maximum end date is 30-jan-2009. Our EST, EFT, LST and LFT should fall between these lines.
Figure: - Forward Calculation
First we need to calculate EST and EFT. EST and EFT are calculated using the forward pass methodology. Figure ‘EST and EFT’ shows how the forward calculation works. We add "0" to the start date i.e. 1-Jan-2009 which becomes the EST of ‘Get Faculties’. ‘Get Faculties’ task takes the 6 days and adds to EST which gives us 7-Jan-2009 which is the EFT for ‘Get Faculties’. EFT becomes the EST of the next task i.e. ‘Buy Computers’. Again we add number of days of ‘Buy Computers’ task to get EFT and so on. In short EFT is calculated by subtracting number of days from EST. EFT of this task becomes the EST of the next task.
Figure: -EST and EFT
Figure: - Backward Calculation
In order to calculate LST and LFT we need to calculate backward from the dead line date.
Figure ‘LST and LFT’ show how the calculation actually happens. As concluded
previously the latest date till which the institute should be ready is 30-Jan-2009. The latest dead line date becomes the LFT of the last activity i.e. ‘Prepare Course’. We subtract the number of days from the LFT and get LST of ‘Prepare Course’ activity which is 20-Jan2009. LST of this activity becomes the LFT of the activity which needs to finish before this i.e. ‘Install Software’ and so on. Figure ‘Backward Calculation’ shows how the calculation moves. LST of the current task is LFT minus number of days. Current Activity LST is then assigned to the previous task’s LFT.
Figure: -LST and LFT
Below figure ‘EST, EFT, LST and LFT’ shows the complete figure after the full
calculation forward and backward.
Note: - Try calculating manually to understand how we reached to every figure in the diagram below. In real time scenario you will be using a tool…So do not worry. But you should understand the concept that’s important.
Figure:- EST, EFT, LST and LFT
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