What is Critical Chain Method in Project Scheduling?

Creating realistic and accurate schedules is the first step towards project success. In Project Scheduling, PERT and Critical Path Method (CPM) are widely known methods. Using CPM, early Start and Finish dates and Late Start and Finish dates are calculated by forward & backward analysis of the project network diagram paths. But this method, does not take resource limitation into consideration. After identifying the path, resources are picked up & leveled. In general, activity owners add safety margin (buffers) to each of the activities in order to cope up with uncertainties. But this cause time waste when activities can be completed well before the estimated finish date.

Critical Chain Method, developed by Dr. Eliyahu M. Goldratt (1997), is a schedule network analysis technique that takes account of task dependencies, limited resource availability & buffers. First step in this method is identifying set of activities that results in longest path to project completion which are called critical chains. As it includes resources into consideration, it may be longer than CPM schedule. Resources used in those critical chain activities are critical resources. Set of activities that are in non-critical chain but converging to critical chain are feeders. Next step is shortening the project schedule by reducing the activity duration estimates with effective buffer management. CCM focuses on eliminating project schedule delays due to uncertainties, overestimation of task duration and wasted internal buffers.

In CPM, even a resource completes an activity before the planned finish date, the time gain is not propagated to next activity as he needs to wait till early start date of next activity. But delays are propagated which may even change the existing critical path. In the below example, I showed activity dependency & resource dependency across activities in a small project.

Any delay in Activity 1 (upper part of fig), will delay the entire project. Also if you see, the critical path is Activity 1 – Activity 3, though activity 2 is completed long before, Activity 3 has to wait till activity 1 is completed.

In the resource-leveled project(lower part of fig), same resource R1 used in two different activities A3 & A2. Though A1 is completed in 2 time units, but A2 has to wait till R1 finishes up A3 which 7 time units. Though A3-A4-Activity 3 is the critical path, but any delay in A3 or A2 will surely delay the entire project & due to Resource dependency another longest path comes into picture A3 – A2 – Activity 3 (15 time units). Hence, optimization of duration is required to concentrate not only on the activity dependencies but also on resource dependency.

Another important factor one need to deal in project scheduling is – safety margins(buffers) to beat the uncertainty. Due to addition of buffers at different activity level, it is very difficult to calculate exact time of project execution & this safety margin will go waste due to Student Syndrome or Parkinson’s Law.

In CCM, as calculation is based on set of activities in the critical chain, hence estimated finish dates for each activity is not of much use. Also, time gain or delay in an activity is fully passed throughout the chain activities. The internal buffers added to tasks are made explicit and summed up. The summed up value is called project buffer. As per CCPM, project buffer is part of project, hence project duration remain unchanged even after stripping off all the activity safety margins. And keeping them improves protection against uncertainties.

Also, 50% probability time estimates are used in calculating the activity durations in CCM instead of traditional 95% probability. Difference between original schedule estimates and new schedule estimates gives the project buffer. In general project scenario, there is 100% chance that activity-wise safety margin get exhausted by poor buffer management. In CCM, pooled project buffer is used as safeguard the entire project & even it is utilized up to 50% allows project completion well within the schedule.

CCM introduces 3 different buffers. 1. Resource buffer, 2. Feeding buffer, 3. Project buffer. Out of these, I discussed about project buffer previously. Let me give gist of other two buffers next.

Resource buffer

Resource buffer is inserted just before critical chain activity where ever a critical resource required. This is used to give a signal to the critical resource that a critical chain activity to which they are assigned is due to start shortly. According to CCPM, this wake-up call will cause the critical resource to wrap up any non-critical work and be ready to start work on the critical chain task as soon as its predecessors are completed. The resource buffer does not actually consume any resource, and it adds neither time nor cost to the project.[2]

Feeding buffer

Same like project buffer, pooling of activity safety margins in a non-critical chain & applying them at the end of those chain.Because this buffer is placed where the path feeds back into the critical chain path, it is called a feeding buffer. See the placement of feeding buffer in the fig.

References:

1.OVERVIEW OF CRITICAL CHAIN PROJECT MANAGEMENT by Francois Retief, Critical Chain Symposium 2002. http://www.hetproject.com/Francois_Retief_paper_Overview_of_Critical_Chain.pdf

2. A CRITICAL LOOK AT CRITICAL CHAIN PROJECT MANAGEMENT By Barnes, Robert, Publication: Project Management Journal, Date: Monday, December 1 2003 http://www.allbusiness.com/management/951030-1.html

3. 1. Critical Chain Project Scheduling By Kailash Awati & Arati Apte. http://www.orafusion.com/pm_cc.htm
4. Critical Chain Project Management Improves Project Performance by Larry P. Leach, Advanced Projects Institute, 1997. http://www.advanced-projects.com/CCPM/PMJOURN_R8.PDF

Brief Overview on Project Time Management Processes

In Project Management, Time Management comes in first place with the project definition. If you see the definition of the project -

A project is a temporary endeavor undertaken to create a unique product, service, or result. [1]

“Temporary” is the key word used to show the importance of Time Management in Project Management. Developing & managing schedule is always challenging for a Project Manager. All activities should be completed within stipulated time to come up with unique result or service. This is what PMBOK says -

“Project Time Management includes the processes required to accomplish timely completion of the project”

Let us review the Time Management processes briefly:

Project Time Management consists of 6 major processes. 5 of them are under Planning Process Group & 1 process comes under Monitoring & Control Process Group.

Activity Definition: Identifying & Defining all specific activities that is needed to produce project deliverables (Planning Process Group)

Activity Sequencing: Identifying dependencies between project activities and documenting the sequence in which they need to be executed to achieve the project output in time (Planning Process Group)

Activity Resource Estimating: Resources are men, materials and machines that are required to execute different project activities. Determining what different type of resource required, in what quantity and when each resource will be available to perform the activities is worked out in this process(Planning Process Group)

Time Management Processes

Activity Duration Estimating: In this process, estimation of required time is done to carry out a defined activity based on the scope & resource availability. Accuracy of this process leads to exact schedule development of whole project & cost estimation of resources (Planning Process Group)

Schedule Development: Coming up with planned start and end dates for every activity & also for the entire project happens in this process. This process uses activity sequences, duration estimation, resource estimation & project schedule constraints to arrive the project schedule. Few well established techniques are already available. Output of this process is approved and used as schedule baseline against which project progress is tracked (Planning Process Group)

Schedule Control: This process explains project status monitoring, finding changes in the schedule(early you find lesser the impact!) and controlling schedule changes to meet planned schedule. Communication is more important in this process (Monitoring & Controlling Process Group)

One can notice high interdependency between time management processes. It is not required that all these processes to be defined separately. In some cases like a project with smaller scope may have all time management processes combined in a single process.

References:
[1]. PMBOK^® Guide – 3rd edition

Resource Leveling

Development of schedule involves analyzing numerous options and applying different techniques to meet the project (imposed) delivery date. Various schedule network analysis techniques are used to generate project schedule. Some of them are Critical path method, critical chain method, what-if analysis and resource leveling. More than one technique can be applied on project schedule to come up with expected schedule.

Estimation of activity resource is a vital process which involves

* Determining what resources will be required (labor, equipment, material)
* What quantity for each resource will be required.
* When are resources available to perform project activities

Resource requirements by time period, often in the form of a resource histogram. Resource histogram is

* A bar chart showing the amount of time assigned to a resource for the time interval.
* Resource availability is depicted as a line for comparison purposes.
* Resources assigned more work than available hours are considered ‘over-allocated’.

If supply is sufficient to meet the demand, then projects does not have any resource allocation issues. But this may not be the case in real time scenario – Demand will exceed supply & resource scarcity arises. Project manager uses a technique called ‘Resource leveling’.

Resource Leveling is a project management technique used to examine a project for an unbalanced use of resources (usually people) over time, and smoothen distribution of resource usage by resolving over-allocations or conflicts. With this technique PM ensures resource demand does not exceed resource availability.

Resource leveling is used:-

* to address schedule activities that need to be performed to meet specified delivery dates
* to address a situation where shared or critical required resources are only available at certain times or are only available in limited quantities
* to keep selected resource usage at a constant level during specific time periods of the project work

Resource leveling is applied to a schedule model that has that has a critical path identified. In critical path method calculation, preliminary early start & late start schedule is calculated which may need more resources in different time line. After resource leveling schedule, projected start and finish dates(which may be longer than the preliminary dates) are calculated to reflect resource availability and even there could be a change in critical path. There are issues with using the simple bar chart for resource leveling as the interdependency between activities is not available in them, which may result incorrect schedule plan.

Generally, there are two approaches to leveling and smoothing the resources required:

* Time-constrained approach – In this case importance will be given on completing the project within a specified (imposed) date. This date will usually have been determined by network analysis. Changes in the duration of any activity, and the resources required at a given time, must be undertaken within the float (slack) available. Obviously there can be no adjustment of activities which are on the critical path.

* Resource-constrained approach – In this approach, the project must be completed with limited available resources even if this means extending the project duration. If the total resource demand exceeds the resource availability at any time then some of the activities must be delayed until there is sufficient resource availability.

For both of the above approaches, information regarding the earliest & the latest start times and slack will be used to level resources.

Few options that can be used in resource leveling are:

* Allocating scarce resources to critical path activities first can be used to develop a project schedule that reflects such constraints.
* Common way to bring the project back on track is resource reallocation from non-critical to critical activities. This may at least bring schedule as close as possible to its originally intended overall duration.
* Resources working extra hours , weekends, or multiple shifts by authorizing overtime to reduce the durations of critical activities.
* Adding or substituting resources of equal or greater productivity to shorten durations that have extended the preliminary project schedule.
* Last but not the least, delaying or extending critical path tasks/task duration.

Other than impact in duration, some of the above options have impact on project cost & risk also.

Three-point estimates

As a manager, you are developing schedule for your project.

How do you specify time estimates for project activities?

From analysis,usually, single-point, most-likely estimates are used for activity duration estimating. But these estimates does not represent accurate information and it may lead to schedule overrun situation.

Project manager’s ultimate goal is to create a realistic project schedule that provides a basis for monitoring project progress for the time dimension of the project. Adapting better estimating technique is needed for project success.

Three-point estimating is one of the general estimating methods that helps project managers produce better estimates. Instead of providing discrete activity duration estimates like 3-weeks, 2-days, etc., the accuracy of it can be improved by considering the amount of risk in the original estimate.

Three-point estimates are based on determining three types of estimates: optimistic, most likely and pessimistic. (i.e. Optimistic – 2 weeks, Most Likely – 4 weeks, Pessimistic – 6 weeks)

* Most likely(approx. realistic scenario): The duration of the schedule activity, given the resources likely to be assigned, their productivity, realistic expectations of availability for the schedule activity, dependencies on other participants, and interruptions.

* Optimistic(best-case scenario): The activity duration is based on a best-case scenario of what is described in the most likely estimate.

* Pessimistic(worst-case scenario): The activity duration is based on a worst-case scenario of what is described in the most likely estimate.

An activity duration estimate can be constructed by using an average of the three estimated durations. Three-point estimates are needed for PERT estimates and Monte Carlo simulations.

The PERT is a more rudimentary approach to quantitative risk analysis. PERT applies a weighted average favoring the most likely outcome: (O + 4M + P)/6

Schedule Network Analysis

Though projects use certain charting techniques like Bar, Milestone, Gantt to represent project schedules, they lack visualization of inter dependencies between different activities.

By constructing schedule networks, we can get inter dependencies between activities and can develop master schedule plan that provides up-to date schedule information about the project. Schedule network analysis is a technique that generates the project schedule.

In short, Schedule Network Analysis is the technique of identifying early and late start dates, as well as early and late finish dates, for the uncompleted portions of project schedule activities. [1]

It employs a schedule model and various analytical techniques, such as critical path method, critical chain method, what-if analysis, and resource leveling to calculate various project schedule information.

Schedule network provides:

* dependencies between activities,

* project completion date,

* analysis of the early and late start and finish dates,

* analysis of what-if scenarios,

* analysis of cost trade-off by crashing the schedule,

* analysis of applying leads and lags,

* analysis of slippage in project schedule

* analysis of scheduled start and finish dates for the uncompleted portions of project schedule activities

* graphical representation of the whole project from start to finish (master schedule plan)

We need to make following considerations before applying any of the analytical techniques

* adjustment should be made on any loops or open ends in schedule network for accurate calculation

* some network paths may have points of path convergence or path divergence that can be identified and used in schedule compression analysis or other analyzes

References:
[1]. PMBOK^® Guide – 3rd edition