Calculation of Float in CPM
Submitted by tweety2010 on Sun, 01/23/2011 - 21:20
Hi,
I am facing an issue in calculating Late Start and Late Finish. I used the backward pass, but I am unable to do the problems in time management. Any explanation about the concept or formulas to calculate Late Start and Late finish would be a great help.
Thanks
Forums:
lijuthomas_80
Mon, 01/24/2011 - 06:58
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Hello,Calculation of LF and
Hello,
Calculation of LF and LS will be very simple, if you follow the proper steps.
Steps to calculate the LS and LF:
1. First you need to do a forward pass to find out the ES and EF (I believe the concept of forward pass is clear to you)
2. Then Identify the critical path
3. Go to the last activity in the critical path, the LF of that activity is the same as EF. (Also the LF of all the last activities in any path is the EF of the last activity in the critical path)
4. Now the formula to calculate LS for that activity = LF - Duration + 1
5. Now you move backwards to the previous activity (Remember you are doing backward pass). The LF of that activity is the LS of the next activity - 1. Here 1 important thing to consider is, while doing backward pass, if more than one activity is merging to one activity, then you must consider the activity which has the lowest LS value
6. LS of that activity is again the same - LF - Duration + 1.
7. Now move on to the next longest path and continue the same process.
Please remember, once you calculated the LS and LF of an activity, do not replace it, even if that activity occurs in a different path.
You need to practice a couple of exercises to be more precise.
tweety2010
Wed, 01/26/2011 - 03:50
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Calculation of LF
Hi Lijuthomas,
Thank you for the explanation. Yes, I am clear about the forward pass and critical path. I don’t know where I am doing wrong but my answers are not tallying with Rita’s examples. Here is one sample example from Rita’s PMP Prep: Please help me in calculating LS and LF:
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Activity 1 can start immediately and has an estimated duration of three weeks.
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Activity 2 can start after Activity 1 is completed and has an estimated duration of three weeks.
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Activity 3 can start after activity 1 is completed and has an estimated duration of six weeks
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Activity 4 can start after activity 2 is completed and has an estimated duration of eight weeks.
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Activity 5 can start after activity 4 is completed and after activity 3 is completed. This activity takes four weeks.
What is float of activity 3, Activity 2 and Activity 5.
I am sharing my calculation for CPM and Forward pass (ES and EF). Please cross check:
CPM:
Path: StartàActivity 1à Activity 2à Activity 4à Activity 5 à End = 3+3+8+4 = 18
Path: Startà Activity 1 à Activity 3à Activity 5 à End = 3+6+4 = 13
Duration of Critical Path is 18 weeks
ES and EF calculation using forward pass:
Activity 1: ES = 0, EF = 3
Activity 2: ES= 3, EF = 6
Activity 4: ES = 6, EF = 14
Activity 3: ES = 3, EF = 9
Activity 5: ES = 14, EF = 18
Here I used the following rules:
ES of first activity in network diagram is ‘0’
ES for all other activities is the latest EF of the previous activity.
EF = ES + duration
Now, to calculate LS and LF for activity 5 (which has dependency on more than one activity i.e. on Activity 4 and Activity 5:
Activity 5: LF = EF = 18
LS = LF – Duration + 1 = 18-4+1 = 15 (but Rita’s answer is 14)
Please help me in explaining this.
Thank you very much.
lijuthomas_80
Thu, 01/27/2011 - 05:28
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Hi,There are multiple ways
Hi,
There are multiple ways for calculating ES,EF,LS and LF. One option is to have the ES of the first activity as 0 or ES of the first activity as 1.
So you selected the first option, ie, ES of the first activity as 0. You did the forward pass perfectly and calculated the ES and EF of all activities as per the first option.
But when you tried the backward pass, you did the calculations as per the second option ie, ES of the first activity as 1, which is wrong.
I will explain:
ES and EF calculation using forward pass (Assuming ES of the first activity as 0):
Activity 1: ES = 0, EF = 3
Activity 2: ES= 3, EF = 6
Activity 4: ES = 6, EF = 14
Activity 3: ES = 3, EF = 9
Activity 5: ES = 14, EF = 18
Now to calculate the LS and LF of Activity5,
LF = EF = 18
LS = LF - Duration = 18-4 = 14 (LS = LF - Duration + 1 would have been correct, if you had opted for the second option ie, ES of the first activity as 1)
Now for Activity 4,
LF = LS of Activity 5 = 14
LS = LF - Duration = 14-8 = 6
Do the same for the rest.
Now let me explain how we can calculate using the second option
ES and EF calculation using forward pass (Assuming ES of the first activity as 1):
Activity 1: ES = 1, EF = 1+3-1 = 4-1 = 3
Activity 2: ES= 3+1 = 4, EF = 4+3-1 = 6
Activity 4: ES = 6+1 = 7, EF = 7+8-1 = 14
Activity 3: ES = 3+1 = 4, EF = 4+6-1 = 9
Activity 5: ES = 14+1 = 15, EF = 15+4-1 = 18
Now to calculate LS and LF
Activity 5 : LF=EF=18, LS = LF-Duration+1=18-4+1=15
Activity 4: LF = LS of successor(in this case, Activity 5) - 1 = 15-1=14, LS = LF-Duration+1 = 14-8+1=7
Do the same for the rest.
Correct me, if I am wrong anywhere.
ammayath1
Thu, 01/27/2011 - 07:16
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very simple...
refer this link for ES/EF/LS/LF: http://www.pmroadtrip.com/pdm_01.html#ES_EF_LS_LF
tweety2010
Fri, 01/28/2011 - 03:44
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Calculation of LF
Thank you for the link. I am not aware of any websites other than PMI. I am following only PMBOK and Rita Mulchy. When I was struggling with doubts, one of my friend referred me this site. Its a great one. Especially forums are really helpful. People are very prompt in response and very informative.
I am happy to meet you.
Thank You - T
tweety2010
Fri, 01/28/2011 - 03:40
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Calculation of LF
Thank you very much. Your explanation is awesome. It is very clear now.
Thank You - T