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Forward pass & Backward pass

6 replies [Last post]
Mon, 2010-01-04 05:50
Shah. HB
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Hi

In CPM algorithm why do we consider max value for float calculation of activities in forward pass and minimum value in backward pass

Please share your thoughts?
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Replies

Fri, 2012-03-09 08:45
#1
Patrick Weaver
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The calculations in some popular software tools are incorrect, particularly float calculations. If you want to understand the correct mathematics see:

http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf

http://www.mosaicprojects.com.au/PDF/Links_Lags_Ladders.pdf

http://www.mosaicprojects.com.au/PDF/Schedule_Float.pdf

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Tue, 2010-01-05 00:05
#2
Anoon Iimos
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Shahul,

I agree with Samer - "The essence is in creating the list of activities, sequence, resources, durations."

Forget Floats, it’s only good during summer!
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Mon, 2010-01-04 22:51
#3
Shah. HB
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Many thanks to all ,let me check the below mentioned procedure and get back to you for further clarification
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Mon, 2010-01-04 11:01
#4
Samer Zawaydeh
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Dear Shahul,

You have an error in your question. The essence is in creating the list of activities, sequence, resources, durations. What is it that you want to know exactly?

It is better to read more about CPM and follow Raviraj and Rafael advice.

With kind regards,

Samer
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Mon, 2010-01-04 07:20
#5
Rafael Davila
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Shahul,

In the forward pass you calculate Early Dates, the date an activity can start, an activity can start after all predecessors are finished so you use the latest date, latest being a maximum value.

In the backward pass you calculate Late Dates, the latest date an activity can finish without delaying project finish date, equals the earliest of all successors latest’s start, earliest being a minimum value.

About float you have finish float as the difference of finish dates and start float the difference of start dates, therefore it is wrong to say an activity has float, is a singular statement, an activity have a start float and a finish float. It might be different under activity splitting and resource constraining,and or under PDM relationships other than FS and or after activities are in progress as activities finish dates might be pulled later by finish relationships or constraints.

As far as I remember P3 would allow you to define it to be either Start or Finish, so you have the option to mislead everyone. I always believed it to be wrong, a reflection of the inability of P3 to fully evolve with the more advanced PDM theory, the same as in the case with float that after resource leveling it means nothing under P3.

MS Project provides you with three definitions for Float with Slack as a synonym of Float:
1) Finish Slack = Late Finish - Early Finish
2) Start Slack = Late Start - Early Start
3) Total Slack = the smaller value of the Late Finish minus the Early Finish field, and the Late Start minus the Early Start field.

I also find MS Project to be misleading in its definition of Total Slack; it is ambiguous and should also be avoided. All software should provide you with both; it can be in a single column a single value if both equals and both values if different, and or on separate columns.

Best regards,
Rafael
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Mon, 2010-01-04 06:00
#6
A D
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Shahul,

Take a small CPM sample and then take max value for fwd pass calculation and then try using max value for backward pass calculation too.

Re-run it again and then u will find the difference.

Do u really need more advise for this?
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