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Forward and Backward Pass in Time Management

07/09/06 | by Clarise Z. Doval Santos [mail] | Categories: Project Management

There are two terms related to Critical Path that one may encounter. These are the terms Forward Pass and Backward Pass. These terms are related to ways of determining the early or late start [forward pass] or early or late finish [backward pass] for an activity.

Forward pass is a technique to move forward through a diagram to calculate activity duration. Backward pass is its opposite.

Early Start (ES) and Early Finish (EF) use the forward pass technique.

To determine the Early Start of an activity, factor in all its dependencies and see its earliest start date.

Consider the following simple diagram (durations are in weeks):
Simple Network Diagram of Forward Backward Pass example
Click to view original size

The Early Start (ES) for Activity B is 4. Why? B comes after A. A starts on week 1 and finishes on week 3. So the earliest that B can start is week 4. For simplicity, I think of it as: The duration of preceding activity + 1

The Early Finish (EF) is the earliest calculated time an activity can end. To calculate Early Finish, (ES for the activity + Activity Duration) - 1. From the diagram above, we can compute the EF of activity B as [(4 + 3) - 1] = 6. Hence, the EF for Activity B is 6.

Late Start (LS) and Late Finish(LF) use the backward pass technique. You can think of backward pass as calculating backward to see how much an activity may slide without affecting the finish date.

Late Start (LS) is the latest time an activity may begin without delaying the project duration. The simplest way one can compute the LS is adding the float to the activity Early Start. Using the simple diagram above, we know that Activity B is on the critical path, hence has a float of zero. Also, Activity B's ES = 4. Hence, LS = (0 + 4) or 4. Note that if an activity has a float of zero, ES and LS will be the same. ;)

Late Finish (LF) latest time an activity may be completedwithout delaying the project duration. One can compute LF by LF =(Activity's LS + Activity Duration) - 1. So the LF of Activity B = (4 + 3) - 1 = 6. Note that since activity B has a zero float, EF = LF.

:idea: For memory trigger, if the float of the activity is zero, the two starts (ES and LS) and the two finish (EF and LF) are the same. Hence, If float of activity is zero, ES = LS and EF = LF.

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This blog contains thoughts that range from non-technical to technical. Its name is derived from "Yakity Blah Blah" a column I once had that discussed a cornucopia of ideas. Who am I? I'm Clarise Z. Doval Santos, providing Project Management and Technical Leadership as part of InterActive Systems & Consulting, Inc.

InterActive Systems & Consulting, Inc. (IASC) performs research in the areas of data analytics, collaboration and remote access.

InterASC Professional Services, a service mark of IASC, provides strategic consulting and project management for data warehousing, business intelligence and collaboration projects using proprietary and open source solutions. We formulate vendor-independent strategies and implement solutions for information management in an increasingly complex and distributed business environment, allowing secure data analysis and collaboration that provides enterprise information in the most valuable form to the right person, whenever and wherever needed.

TeleInterActive Networks, a service mark of IASC, hosts open source applications for small and medium enterprises including CMS, blogs, wikis, database applications, portals and mobile access. We provide the tools for SME to put their customer at the center of their business, and leverage information management in a way previously reserved for larger organizations.

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