Download and which activities are on the critical path? PM: Scheduling

Project Management
Project Time Management
(Scheduling)
Minder Chen, Ph.D.
CSU Channel Islands
[email protected]
Time
• Limited time is the one constraint of any project
with which we are all probably most familiar.
• For many projects that create a product or
event, time is the most important constraint to
manage.
• Project team members might not know the
project budget or the scope of work in great
detail, but chances are they all know the project
deadline.
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Sample Time Constraints
• You are building a house and must finish the
roof before the rainy season arrives.
• You are assembling a large display booth for a
trade show that starts in two months.
• You are developing a new inventory-tracking
system that must be tested and running by the
start of the next fiscal year.
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Project Time Management
• 6.1 Plan Schedule Management—The process of establishing the policies,
procedures, and documentation for planning, developing, managing,
executing, and controlling the project schedule.
• 6.2 Define Activities—The process of identifying and documenting the
specific actions to be performed to produce the project deliverables.
• 6.3 Sequence Activities—The process of identifying and documenting
relationships among the project activities.
• 6.4 Estimate Activity Resources—The process of estimating the type and
quantities of material, human resources, equipment, or supplies required to
perform each activity.
• 6.5 Estimate Activity Durations—The process of estimating the number of
work periods needed to complete individual activities with estimated
resources.
• 6.6 Develop Schedule—The process of analyzing activity sequences,
durations, resource requirements, and schedule constraints to create the
project schedule model.
• 6.7 Control Schedule—The process of monitoring the status of project
activities to update project progress and manage changes to the schedule
baseline to achieve the plan.
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Scheduling Overview
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Examples of Project Schedules
CPM: critical path method
PERT: Project Evaluation &
(Gantt Chart)
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Review Technique
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Project Schedule — Graphic Examples
•
•
Milestone charts. These charts are similar to bar charts, but only identify the scheduled start or
completion of major deliverables and key external interfaces.
Bar charts. These charts, also known as Gantt charts, represent schedule information where
activities are listed on the vertical axis, dates are shown on the horizontal axis, and activity
durations are shown as horizontal bars placed according to start and finish dates.
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Project Schedule—Graphic Examples
Project schedule network diagrams. These diagrams are commonly presented
in the activity-on-node diagram format showing activities and relationships without a
time scale, sometimes referred to as a pure logic diagram or presented in a timescaled schedule network diagram format that is sometimes called a logic bar
chart.
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Rules of performance measurement
Earned value management (EVM) rules or other
physical measurement rules of performance
measurement are set. For example, the schedule
management plan may specify:
• Rules for establishing percent complete,
• Control accounts at which management of progress
and schedule will be measured,
• Earned value measurement techniques (e.g.,
baselines, fixed-formula, percent complete, etc.) to be
employed
• Schedule performance measurements such as
schedule variance (SV) and schedule performance
index (SPI) used to assess the magnitude of variation
to the original schedule baseline.
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Rolling Wave Planning
• Rolling wave planning is an iterative planning
technique in which the work to be accomplished in
the near term is planned in detail, while the work in
the future is planned at a higher level. It is a form of
progressive elaboration.
• Therefore, work can exist at various levels of detail
depending on where it is in the project life cycle.
• During early strategic planning, when information is
less defined, work packages may be decomposed
to the known level of detail. As more is known
about the upcoming events in the near term, work
packages can be decomposed into activities.
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Milestone
• A milestone is a significant point or event in a
project. A milestone list is a list identifying all
project milestones and indicates whether the
milestone is mandatory, such as those required by
contract, or optional, such as those based upon
historical information.
• Milestones are similar to regular schedule activities,
with the same structure and attributes, but they
have zero duration because milestones represent
a moment in time.
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Sequence Activities
Activity A is the predecessor of Activity B.
Activity B is the successor of Activity A.
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Precedence Diagramming Method
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Dependency
• Finish-to-start (FS). A logical relationship in which a successor activity
cannot start until a predecessor activity has finished. Example: The
awards ceremony (successor) cannot start until the race (predecessor)
has finished.
• Finish-to-finish (FF). A logical relationship in which a successor
activity cannot finish until a predecessor activity has finished. Example:
Writing a document (predecessor) is required to finish before editing the
document (successor) can finish.  Editing task can start before
Writing is finished.
• Start-to-start (SS). A logical relationship in which a successor activity
cannot start until a predecessor activity has started. Example: Level
concrete (successor) cannot begin until pour foundation (predecessor)
begins.
• Start-to-finish (SF). A logical relationship in which a successor activity
cannot finish until a predecessor activity has started. Example: The first
security guard shift (successor) cannot finish until the second security
guard shift (predecessor) starts.
In PDM, finish-to-start is the most commonly used type of precedence
relationship. The start-to-finish relationship is very rarely used.
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Dependency Determination
• Mandatory dependencies. Mandatory dependencies are those that are
legally or contractually required or inherent in the nature of the work.
• Discretionary dependencies. Discretionary dependencies are sometimes
referred to as preferred logic, preferential logic, or soft logic. Discretionary
dependencies are established based on knowledge of best practices within a
particular application area or some unusual aspect of the project where a
specific sequence is desired, even though there may be other acceptable
sequences.
• External dependencies. External dependencies involve a relationship
between project activities and non-project activities. These dependencies are
usually outside the project team’s control.
• Internal dependencies. Internal dependencies involve a precedence
relationship between project activities and are generally inside the project
team’s control.
External Internal
Mandatory
Discretionary
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Leads or Lags
A lead is the amount of time whereby a successor activity can be
advanced with respect to a predecessor activity. For example, on a
project to construct a new office building, the landscaping could be
scheduled to start two weeks prior to the scheduled punch list completion.
This would be shown as a finish-to-start with a two-week lead.
SS + 15 Days(Lag)
A lag is the amount of time whereby a successor activity will be delayed with
respect to a predecessor activity. For example, a technical writing team may
begin editing the draft of a large document 15 days after they begin writing it.
This can be shown as a start-to-start relationship with a 15-day lag.
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Precedence Diagramming Method
• Visually easier to see precedence relationships
• Ideal for large projects with many activities
• They consist of a network of branches and nodes.
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Developing the Project Plan
• The Project Network is a diagram that graphically
depicts the sequence, interdependencies, and start
and finish times of the project job plan of activities
(tasks).
–
–
–
–
–
–
Provides the basis for scheduling labor and equipment.
Enhances communication among project participants.
Provides an estimate of the project’s duration.
Provides a basis for budgeting cash flow.
Identifies activities that are critical.
Highlights activities that are “critical” and can not be
delayed.
– Help managers get and stay on plan.
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Project Network Terminology
• Activity:
– an element of the project that
requires time.
A
• Merge Activity:
– an activity that has two or more
preceding activities on which it
depends.
• Parallel (Concurrent) Activities:
B
D
C
– Activities that can occur
independently and, if desired,
not at the same time.
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Project Network Terminology
• Path: a sequence of connected, dependent
activities.
• Critical path: It is the longest path through the
activity network that allows for the completion of
all project-related activities; the shortest expected
time in which the entire project can be completed.
Delays on the critical path will delay completion of
the entire project.
C+B+D=2+4
A+B+D=3+4
C
B
D
A
(Assumes that minimum time required to complete A + B > minimum
time required to complete C + B; A B  D is the critical path)
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Basic Rules to Follow in Developing Project Networks
1. Networks typically flow from left to right.
2. An activity cannot begin until all preceding
connected activities are complete.
3. Arrows indicate precedence and flow
and can cross over each other.
4. Each activity must have a unique identify
number that is greater than any of its
predecessor activities.
5. Looping is not allowed.
6. Conditional statements are not allowed.
7. Use common start and stop nodes.
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Project Network Terminology
• Event: a point in time when an activity is
started or completed. It does not consume
time.
• Burst Activity:
– an activity that has more than one activity
immediately following it (more than one
dependency arrow flowing from it).
• Two Approaches
A
B
C
– Activity-on-Node (AON)
» Uses a node to depict an activity.
D
– Activity-on-Arrow (AOA)
» Uses an arrow to depict an activity.
• AON is the method used by most project
management software packages.
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Activity-on-Arrow (AOA)
• Identify the critical path
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Critical Path
• Project Management Network Diagrams are
helpful in determining where most of the
schedule project risks (for project delay) will
occur.
• A critical path is made up of activities that
cannot be delayed without delaying the end
date of the project.
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Simple Example of Critical Path Method
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Create a List of Activity
Create the dependency diagram from the list above!
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Koll Business Center—Complete Network Sample
List all activities and connect them based on their dependencies.
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Koll Business Center—Complete Network
6-9
Indicate the duration (how many days) for each task.
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Project Activity Notations
ES: Early Start
ID
EF: Early Finish
SL: Slack or Float
Description: Activity Name
LS: Late Start
Dur: Duration
LF: Late Finish
The activity ID and name
The normal duration time (DUR)
The early start time (ES)
The early finish time (EF)
The late start time (LS)
The late finish time (LF)
The slack
http://en.wikipedia.org/wiki/Critical_path_method
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Network Computation Process
• Forward Pass—Earliest Times
– How soon can the activity start? (early start—ES)
– How soon can the activity finish? (early finish—EF)
– Derive how soon can the whole project finish?
(expected time—ET)
• Backward Pass—Latest Times
– How late can the activity start? (late start—LS)
– How late can the activity finish? (late finish—LF)
• Calculate how long can activity be delayed? (slack
or float—SL)
• Identify critical paths (one or more) and which
activities are on the critical path?
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Forward Pass: Earliest Time
Start
from
day 0
How soon can the project finish?
Early Start (ES) + Duration = Early Finish (EF) or
Early Start (ES) + Duration – 1 = Early Finish (EF)
ES of an activity = the largest EF of its immediately preceding activities
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Forward Pass Computation
• Add activity times along each path in the
network (ES + Duration = EF).
• Carry the early finish (EF) to the next
activity where it becomes its early start
(ES) unless…
• The next succeeding activity is a merge
activity, in which case the largest EF of
all preceding activities is selected to be
the ES of this activity.
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Backward Pass: Latest Time
Latest day
that the
project
should start
Start here: The latest days that
the project has to finish.
Late Finish (LF) - Duration = Late Start (LS) or
Early Start (ES) + Duration = Early Finish (EF)
LF of an activity = the smallest LS of its immediately successsor activities
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Backward Pass Computation
• Use the EF of the last activity to be the LF
of the last activity to get started.
• Subtract activity times along each path in
the network (LF - Duration = LS).
• Carry the late start (LS) to the next
proceeding activity where it becomes its
late finish (LF), unless…
• The next succeeding activity is a burst
activity, in this case the smallest LS of all
its intermediate successor activities is
selected to establish its LF.
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Slack and Slippage
• Now find the slack time (or 'float') for each task by
subtracting the early start from the late start.
SL (Slack) = LS – ES
Or substrcting early start from the late finish
SL (Slack) = LF – EF
• The slack time is the amount of time the task can be
slipped by without affecting the end date of the
process.
• The critical path can now be identified as all paths
through the network where the slack time is zero.
• Slippage: The amount of time that a task has been delayed
from its original baseline plan. The slippage is the
difference between the actual start or finish date for a task
and the baseline (scheduled) start or finish date.
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Backward Pass with Slack
• Which path is the critical path?
165
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Determining Free Slack (or Float)
• Free Slack or just slack time: how much a task can
slip before it delays another task.
– It is the amount of time an activity can be delayed after
the start of a longer parallel activity or activities.
– It is how long an activity can exceed its early finish date
without affecting early start dates of any successor(s).
– Allows flexibility in scheduling scarce resources.
• Total slack is how much a task can slip before it delays the
whole project.
• An activity on critical path has "zero free float", but activity
that has zero free float might not be on the critical path.
• Total float is associated with the path. The total float of a
path is the combined free float values of all activities in a
path.
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Sensitivity
• Sensitivity
– The likelihood the original critical path(s) will
change once the project is initiated.
– The critical path is the network path(s) that has
(have) the least slack in common.
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Develop Schedule: Tools and Techniques
•
•
•
•
•
•
•
Schedule Network Analysis
Critical Path Method
Critical Chain Method
Resource Leveling
What-if Scenario Analysis
Applying Leads and Lags / Laddering
Schedule Compression Techniques:
– Crashing the critical path (i.e., shortening the
durations of critical path activities by adding
resources).
– Fast tracking (i.e., performing more activities in
parallel)
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Critical Path Method (CPM)
• In project management, a critical path is the
sequence of project network activities which
add up to the longest overall duration.
• This determines the shortest time possible to
complete the project.
• Any delay of an activity on the critical path
directly impacts the planned project completion
date i.e. there is no float (slack) on the critical
path.
• A project can have several, parallel, near critical
paths.
http://en.wikipedia.org/wiki/Critical_path_method
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Critical Path Analysis
• What tasks must be carried out.
• Where parallel activity can be performed.
• The shortest time in which you can complete a
project.
• Resources needed to execute a project.
• The sequence of activities, scheduling and
timings involved.
• Task priorities.
• The most efficient way of shortening time on
urgent projects.
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Critical Path Analysis
• Regularly view the critical path(s).
• Closely monitor critical tasks.
• Review series of tasks that may become
part of the critical path.
• Protect yourself by examing tasks that
can slip without affecting the critical path.
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PERT (Program Evaluation and Review Technique)
• PERT is a variation on Critical Path Analysis
that takes a slightly more skeptical view of time
estimates made for each project stage.
• To use it, estimate the shortest possible time
each activity will take, the most likely length of
time, and the longest time that might be taken if
the activity takes longer than expected.
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Three-Point Estimates
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CPM vs. PERT
• CPM or "Critical Path Method":
– A single estimate for activity time was used
that did not allow for variation in activity times
– Activity times are assumed to be known or
predictable ("deterministic")
• PERT (Project Evaluation and Review
Technique)
– Multiple time estimates were used for each
activity that allowed for variation in activity
times
– Activity times are assumed to be random, with
assumed probability distribution
("probabilistic")
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How to Create a Schedule from a Precedence Diagram
Resource
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Practical Considerations
• Network logic errors
• Activity numbering
• Use of computers to
develop networks
• Calendar dates
• Multiple starts and
multiple projects
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Accelerating Project Completion
• When resources are not constrained
–
–
–
–
–
Add resources
Outsource some project work
Schedule overtime
Establish a core project team
Do it twice (quick prototype to get by and than the
real one)
• When resources are constrained
– Fast-tracking
– Reduce project scope
– Compromise quality (rarely used)
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Schedule Compression Techniques
• Crashing: A technique used to shorten the schedule duration
for the least incremental cost by adding resources. Examples
of crashing include approving overtime, bringing in
additional resources, or paying to expedite delivery to
activities on the critical path. Crashing works only for
activities on the critical path where additional resources will
shorten the activity’s duration. Crashing does not always
produce a viable alternative and may result in increased
risk and/or cost.
• Fast tracking. A schedule compression technique in which
activities or phases normally done in sequence are
performed in parallel for at least a portion of their duration.
An example is constructing the foundation for a building
before completing all of the architectural drawings. Fast
tracking may result in rework and increased risk. Fast
tracking only works if activities can be overlapped (even
partially overlapped) to shorten the project duration.
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Extended Network Techniques
• Laddering (further decomposition)
– Activities are broken into segments so the
following activity can begin sooner and not
delay the work.
• Lags
– The minimum amount of time a dependent
activity must be delayed to begin or end.
» Lengthy activities are broken down to reduce the
delay in the start of successor activities.
» Lags can be used to constrain finish-to-start, startto-start, finish-to-finish, start-to-finish, or
combination relationships.
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Use of Lags
Finish-to-Start Relationship
Start-to-Start Relationship
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Precedence Diagramming Method
• The project management team determines the dependencies that may
require a lead or a lag to accurately define the logical relationship.
• A lead allows an acceleration of the successor activity.
• A lag directs a delay in the successor activity.
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Use Lead or Lag to Create Partially Parallel Tasks
• Fast Tracking
Overlapped
Overlapped
SS + 15 Days (Lag)
Lead Time - is an acceleration of the
successor activity
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Lag Time - is a delay imposed
on the relationship between
two activities
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Hammock Activities
• An activity that spans over a segment of a project.
Duration of hammock activities is determined after the
network plan is drawn.
• Hammock activities (summary activity) are used to
aggregate sections of the project to facilitate getting the
right amount of detail for specific sections of a project.
http://en.wikipedia.org/wiki/Hammock_activity
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Hammock Activity
A hammock activity can group tasks which are not
related in the hierarchical sense of a Work
Breakdown Structure, or are not related in a logical
sense of a task dependency where one task must
wait for another. Usages include:
• Group dissimilar activities that lead to an overall
capability, such as preparations under a summary
label, e.g. "vacation preparation";
• Group unrelated items for the purpose of a
summary such as a calendar-based reporting
period, e.g. "First quarter plans";
• Group ongoing or overhead activities that run the
length of an effort, e.g. "project management"
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Avoid Linking Summary Tasks
It generally is not recommended to use links on summary
activities because the logic can be difficult to follow and
the practice may not be supported by all scheduling tools.
Use of links on summary activities may produce logic
errors and create circular logic within the schedule
model.
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Resource Leveling and Resource Smoothing
http://www.izenbridge.com/blog/underlining-the-differencesbetween-resource-leveling-and-resource-smoothing/
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Resource Leveling
• A technique in which start and finish dates are
adjusted based on resource constraints with the goal
of balancing demand for resources with the available
supply.
• Resource leveling can be used when shared or
critically required resources are only available at
certain times, or in limited quantities, or overallocated, such as when a resource has been
assigned to two or more activities during the same
time period or to keep resource usage at a constant
level.
• Resource leveling can often cause the original
critical path to change, usually to increase.
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Resource Smoothing
• A technique that adjusts the activities of a schedule
model such that the requirements for resources on
the project do not exceed certain predefined
resource limits.
• In resource smoothing, as opposed to resource
leveling, the project’s critical path is not
changed and the completion date may not be
delayed.
• In other words, activities may only be delayed
within their free and total float. Thus resource
smoothing may not be able to optimize all
resources.
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Key Terms
Activity
Activity-on-arrow (AOA)
Activity-on-node (AON)
Burst activity
Concurrent engineering
Critical path
Early and late times
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Gantt chart
Hammock activity
Lag and Lead relationship
Merge activity
Network sensitivity
Parallel activity
Slack/float—total and free
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Free Float vs. Total Float
http://en.wikipedia.org/wiki/Float_(project_management)
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Extra
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Critical Chain Method
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Critical Chain Method
• The resource-constrained critical path is known as the critical chain.
The critical chain method adds duration buffers that are non-work
schedule activities to manage uncertainty. One buffer, placed at the end
of the critical chain, is known as the project buffer and protects the
target finish date from slippage along the critical chain. Additional
buffers, known as feeding buffers, are placed at each point that a chain
of dependent tasks not on the critical chain feeds into the critical chain.
• Feeding buffers thus protect the critical chain from slippage along the
feeding chains.
• The size of each buffer should account for the uncertainty in the
duration of the chain of dependent tasks leading up to that buffer. Once
the buffer schedule activities are determined, the planned activities are
scheduled to their latest possible planned start and finish dates.
• Consequently, in lieu of managing the total float of network paths, the
critical chain method focuses on managing remaining buffer durations
against the remaining durations of task chains.
Ref: http://en.wikipedia.org/wiki/Critical_chain_project_management
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