Apply Theory Of Constraints (TOC) and
provide time buffers for any unforeseen delays in projects
Apply TOC to eliminate
safety time
Conventional project management technique
provides safety time for individual activities in a project.
The conventional approach focuses on successful on-time
completion of each individual activity in a project. However,
the Theory of Constraints (TOC) approach focuses on successful
on-time completion of the entire project. According to TOC,
the main constraint in any project is the time taken for
completion of the critical chain. Therefore emphasis is
laid on completing activities in the critical chain without
wasting any time. Hence, cutting safety time from individual
activities eliminates the major cause of time wastage, thereby
removing the constraint.
However, this does not mean that the project
is to be left unprotected against any unforeseen delays
in any individual activity. The project is to be guarded
against delays by providing time buffers. Four different
kinds of time buffers provided to guard the project are:
These buffers provide the necessary cushioning
to individual activities to enable them to accomplish the
final goal of completing the critical chain in the least
possible time.
Critical chain in a project is defined as
the longest sequence of activities from several different
paths, connected by activities performed by common resources.
The safety time provided for the critical chain is defined
as the project buffer. The safety time is removed from individual
activities and is utilised to create the project buffer.
Critical chain activities, which take less than the median
time for completion, partially compensate for activities,
which take longer than median time. The project buffer absorbs
any further delays.
PERT,PROJECT BUFFER
& FEEDING BUFFER
In the PERT (Programme Evaluation and Review
Technique) chart shown in figure 1 the alphabets inside
the circles represent the activities and the numerals on
top of the circles represent the estimated time of each
activity. The activities connected by arrows shown in bold
indicate the critical chain.
It is clear from the chart that the minimum
time required to complete the project is 56 days. Even a
very conservative estimate will indicate that the safety
time for each activity is about half of its estimated time.
If the safety time from each activity is removed, the project
duration is reduced to 28 days. As a thumb rule, it is suggested
to add about half of the removed safety time to the critical
chain as project buffer. In the example, 14 days are added
to the critical chain as project buffer. The revised
project completion time is 42 days, which is 14 days less
than the original estimate.
Feeding buffer is the extra time provided
for activities on the non-critical path, which merge with
the critical chain. The feeding buffer provides a guard
against delays in non-critical paths affecting the critical
chain. The feeding buffer, as a thumb rule is normally assumed
half of the time removed from activities on the feeding
path. The feeding path CF merging into activity I in the
PERT chart in figure 1, sets free 8 days of safety time.
Therefore, the feeding buffer is taken as 4 days. Activity
F should be scheduled to be finished 4 days before activity
I is to begin. The feeding buffer does not increase the
project completion time. If at all the feeding buffer is
inadequate, resulting in delays in critical chain activities,
the project buffer absorbs it.
RESOURCE BUFFER &
CONSTRAINT BUFFER
The Resource buffer guarantees the
availability of resources required for activities on the
critical chain. Providing scheduled idle time for the resource
establishes the resource buffer. Another way is to send
timely updates to the resource manager of the project
progress, so that the resource is available to perform the
critical chain activity as scheduled. Critical chain activity
can therefore begin even before its scheduled time, if required.
This early start can either shorten the project completion
time or compensate for any delays in the critical path.
A constraint buffer is established
to maintain the schedule of a constraint resource handling
multiple projects. It ensures that all the required project
activities are completed, before the constraint resource
begins a scheduled specific activity. The scheduling becomes
complicated when there are several constraint resources
handling multiple projects. It is advisable to use specialised
software to efficiently resolve the resource sharing problem.
Projects involve a high level of uncertainty
and depend heavily on the contributions of individuals.
Project manager needs to work with different departments
involved in the project to estimate lead times so that they
meet the needs of the critical chain. The critical chain
concept starts with a set of talented and driven project
managers and assumes that the resource constraints are within
the scope of the project but not in its leadership. In
truth, leadership may be the larger constraint.
Related Reading: Manage your projects
for success: An application of the theory of constraints.
Umble, M.; Production and Inventory Management Journal,
06/2000.
