- Building a dynamic CPM schedule
- CPM time analysis calculations
- The critical path and float
- Useful External Web-links &
Resources.
Other related sections of the PMKI:
- Resource & Costs
Analysis
- Reporting &
Communicating Controls Information
The development of CPM in the 1950s was limited by the limited processing power of the computers then available. These limitations can have significant impacts on the CPM modelling process we still use today. Given these constraints, designing a schedule for a complex project is an art, the project needs to be 'planned' before an effective schedule can be developed; WP1039 focuses on planning.
DP: A Guide to Scheduling Good Practice describes the work undertaken by a scheduler to create an effective 'dynamic schedule' and is consistent with the PMBOK® Guide and the PMI Practice Standard for Scheduling. Open the paper.
DP: Dynamic Scheduling describes the benefits derived from developing a 'good schedule' as defined in a 'A Guide to Scheduling Good Practice'. Open the paper.
WP: Rolling Wave Planning. Rolling wave is a form of progressive elaboration, increasing the detail in a schedule as more information becomes available.
WP: Schedule Density. The concept of schedule density is similar to rolling wave planning but applies a time based three stage approach to developing an overall summary, 12 month intermediate, and 3 month detailed schedule.
DP: Links, Lags & Ladders focuses on the issues, problems and challenges of overlapping tasks in a schedule using various link types (F-S, S-S, F-F-, S-F) with or without lead and lag durations, the more sophisticated 'ladder' concept, and the importance of actively managing the 'space' between tasks. Open the paper.
Blg: CPM Scheduling – the logical way to error #1. Logical anomalies in a CPM schedule can cause unexpected consequences when an activity duration is changed - for example, increasing the duration of a critical activity reduces the overall project duration. This post highlights the issue.
Art: Hammocks, LOE and Summary Activities in Schedules. Hammocks, LOE and Summary Activities are three completely different types of activity – the differences are defined.
Duration Estimating
Probably the most common
action undertaken by planners everywhere is assigning a
duration to a task; most of us do this almost
automatically. Generally it is only when a dispute arises
the complex interaction of the factors discussed in this
sub-section come into play. The purpose of this section is
not to reach any conclusion on what is the ‘correct’
way to assess durations (we suggest there is no
universally correct answer this question), rather to put
into one place the multiple factors that should be
considered by competent planners and managers. The initial
planning decisions concern the project framework used to
define the task:
Once the task is determined, and the overall project framework set; the issues concerning the estimation of the optimum task duration come into play:
WP: Duration Estimating. The challenge of assigning an accurate duration to a task. This paper describes the processes involved in duration estimating. The important effect of cognitive biases on the way estimates are developed is discussed in WP1069.
PP: (The) Cost of Time - or who's duration is it anyway? Probably the most common action undertaken by schedulers everywhere is assigning a duration to a task. This paper outlines the factors influencing the choice of an optimum duration. Considers the factors that can cause the duration to be modified during the planning phase and then outlines some of the likely costs associated with accelerating the project. Download the paper.
Prs: Estimating Fallacies - excessive detail does not help. Estimating costs and durations can be done in great detail, however, detailed is not synonymous with accurate! This paper sets out a pragmatic framework for estimating that offers realistic levels of accuracy to generate sensible expectations for a reasonable investment of estimating effort. Download the presentation.
WP: Schedule Compression. This WP focuses on the techniques and risks associated with schedule compression, including 'fast-tracking' and 'crashing'.
DP: Schedule Calculations a guide the Time Analysis and Float calculations used in PDM schedules. Open the paper.
Blg: Schedule Calculations – Old and New. The difference between old manual calculations starting from Zero, and the correct calculations used by modern computers.
The mathematical precision of critical path scheduling also caught the attention of lawyers and contract draftsmen leading to the evolving concept of the ‘contract program’ and conferring a degree of legal certainty onto the schedule that cannot be supported by objective analysis. This topic analyses the factors creating the ‘critical path’ and ‘float’ within a schedule and then look at ways of resolving the conflicting views around the critical path and float.
WP: Defining the Critical Path. There are many different descriptions in regular use, this WP provides a concise and accurate definition.
Blg : Critical confusion – when activities on the critical path don’t compute…… The use of Finish-to-Finish and Start-to-Start links (particularly in combination) can cause significant issues in calculating the overall project duration.
The
concept of schedule ‘float’ was created in 1957. The
existence of a ‘critical path’ and non-critical activities
(with their associated ‘float’) grew out of the science of
‘scheduling’ as defined by Critical Path Analysis (CPA).
In 1956/57 Kelley and Walker started developing the
algorithms that became the ‘Activity-on-Arrow’ or ADM
methodology and from the 1960s onward, the use of
‘critical path’ techniques began to dominate project
scheduling; see: A Brief History of Scheduling - Back to
the Future.
During the 60s and 70s significant advances in the concept of float occurred as the ADM scheduling methodology developed. Much of this sophistication has been lost in the intervening 50 years as the PDM methodology gained precedence. How significant is this loss of insight? From a practical perspective there are two issues of paramount importance: Resources leveling and smoothing is completely reliant on having access to accurate and understandable float values. The absence of these means the scheduling algorithms are likely to be less efficient.
Contract management relies on clearly defining critical and non-critical activities and knowing how much flexibility (float) is reasonably available on the non-critical activities. This topic argues that the lack of defined calculations for most of the float values in a PDM schedule must reduce the overall value of the schedule model compared to more rigorous approaches. But also recognizes that if scheduling is a modelling process designed to affect the future behaviours of people working on the project (ie, persuade them to work to the plan), other factors may be more important.
DP: Schedule Float discusses the various calculations and definitions for float in both ADM and PDM networks. Open the paper.
Art: Calculating and Using Float. Based on the above, this review looks at the different types of float and slack that used to be calculated in ADM and PDM schedules, compared to the limited options used today (see the original PM World Today article).
PP: Float - Is It Real? The existence of a ‘critical path’ and non-critical activities (with their associated ‘float’) grew out of the science of ‘scheduling’ as defined by Critical Path Analysis (CPA) and are barely 50 years old! This paper analyses the factors creating the ‘critical path’ and ‘float’ within a schedule and then look at ways of resolving the conflicting views of float encountered in the literature. Potential solutions include the UK ‘Delay and Disruption Protocol’, client led integrated teams and the use of alternative planning methods such as location based scheduling, trend analysis and earned schedule. Download the paper.
Art: Float is it real? Float only exists because of the limitations built into CPM calculations, while it is useful, it is not 'real' - short summary.
Access the Guild of Project Controls Body of Knowledge. A suite of process-based documents which define Project Controls (membership required): http://www.planningplanet.com/guild/GPCCAR-modules
