PROJECT TIME MANAGEMENT
7.5 Schedule development
7.5.1
Gantt charts
Henry Gantt developed the first Gantt chart during World War I for scheduling work in factories. Early versions of Gantt charts normally do not show relationships among projects, which is a major disadvantage. However, project management software such as Microsoft Project® can assist in creating more sophisticated Gantt charts that can
also show dependencies in some format, but not as clearly as they would be displayed on a network diagram.
Figure 7.3 is a typical Gantt chart for a software development project as shown in
Microsoft Project®.
As noted in Figure7.3, there are different symbols used on the Gantt chart:
• Milestone: black diamond symbol that represents a significant event with zero duration
• Summary tasks: thick black bars
• Individual tasks: light gray horizontal bar
• Relationships or dependencies between tasks: arrows connecting the task sym- bols.
Milestone tasks are those with zero duration. TheSMART criteria are used as guide- lines to identify milestones, meaning that milestones should be:
• specific • measurable • assignable • realistic • time-framed.
A tracking Gantt chart is a special Gantt chart that compares planned and actual project schedule information. The planned schedule dates for activities are called the
baseline dates. Figure7.4depicts a tracking Gantt chart.
As a progress evaluation tool, tracking Gantt charts have a few additional symbols:
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Each task often has two horizontal bars. The top one represents the planned or baseline duration for each task, whereas the bar below it represents the actual information.Ë
A striped horizontal bar represents the planned duration for summary tasks. The black bar adjoining it shows progress for summary tasks.Ì
A white diamond on the tracking Gantt chart represents aslipped milestonethat is completed later than originally planned.Í
Percentages to the right of the horizontal bars display the percentage of work completed for each task.Compared to the network diagram, the main disadvantageof Gannt charts lies in the fact that Gantt charts do not usually show dependencies between tasks. However, project management software packages such asMicrosoft Project® can create Gantt
charts with the dependencies displayed, provided tasks are linked. Nevertheless, the dependencies are not shown as clearly as they would be displayed on a network dia- gram.
Detailed interpretations on how Gantt charts and tracking Gantt charts work can be found on Appendix A in Schwalbe’s text, or by consultingMicrosoft Project® help doc-
uments.
7.5.2
Critical path method
Critical path method (CPM), also known ascritical path analysis, is a project network analysis technique used to predict total project duration. It is an important tool that helps project managers combat project schedule overrun.
Acritical path for a project is the series of activities (tasks) that determine the earliest time by which the project can be completed. It is thelongest paththrough the project network diagram. Slack or float is the amount of time an activity may be delayed without delaying a succeeding activity or the project finish date.
Given a network diagram, how to calculate the critical path involves adding the dura- tions for all activities on each path through the network diagram. The longest path is the critical path. However, the critical path represents theshortest time it takes to complete a project.
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The wordcriticaldoes not indicate that the critical path includes all critical activi- ties. Rather, it is only concerned with the time dimension of a project.Ë
Although the critical path is the longest path, it represents the shortest time it takes to complete the project, because each activity must be done to complete the project.Ì
There can be two or more critical paths on a project instead of just one.Í
The critical path on a project can change as the project progresses. It does not have to be static.Some important concepts related to the critical path method are summarised as fol- lows:
• Early start date: the earliest possible time an activity can start based on the project network logic.
• Early finish date: the earliest possible time an activity can finish based on the project network logic.
• Late start date: the latest possible time an activity might begin without delaying the project finish date.
• Late finish date: the latest possible time an activity can be completed without delaying the project finish date.
• Free slack orfloat is the amount of time that an activity can be delayed without delaying the early start date of any immediately following activities.
• Total slack orfloat is the amount of time that an activity can be delayed from its early start without delaying the planned project finish date.
The early start and finish dates for a project can be determined by aforward passof the project network diagram. Likewise, the late start and finish dates for a project can be determined by abackward passof the project network diagram. One need to following the following rules for the forward and backward passes. Note this information is not available in Schwalbe’s text, but taken from chapter 6 in Hughes and Cotterell’s text.
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Forward pass rule: the earliest start date for an activity is the earliest finish date for the preceding activity. Where there is more than one immediately preceding activity, one should take thelatest of theearliest finish dates for those activities.Ë
Backward pass rule: the latest finish date for an activity is the latest start date for all the activities that may commence immediately that activity is complete. Where more than one activity can commence, one should take theearliestoflatest start datesfor those activities.The significance of identifying critical paths of a project is that project managers can reduce the duration of the project by shortening the duration of critical path activities using duration compression techniques such as crashing andfast tracking. Table7.1
Table 7.1: Crashing and fast tracking
Technique What It Is Main Disadvange
Crashing A technique for making cost and schedule trade-offs to obtain the greatest amount of schedule com- pression for the least incremental cost
Often increases total project costs
Fast tracking
Involves doing activities in parallel that one would normally do in se- quence.
It can end up lengthening the project schedule since start- ing some tasks too soon of- ten increases project risk and results in rework
7.5.3
Critical chain scheduling
Critical chain scheduling is another technique, which is based on thetheory of con- straints to meet project complete schedule. The theory of constraints (TOC), a man- agement philosophy developed by Eliyahu M. Goldratt, is based on the fact that any complex system at any point in time often has only one aspect or constrain that limits its ability to achieve more of its goal, like a chain with its weakest link. Critical chain scheduling is a method of scheduling that considers limited resources when creating a project schedule and includes buffers to protect the project completion date.
As a comparison to critical path method that does not consider resource allocation, critical chain scheduling takes the availability of resources into consideration when scheduling projects.
Multitasking occurs when a resource works on more than one task at a time. Multi- tasking often wastes setup time, increases total project duration and is thus avoided by critical chain scheduling. The examples in Figures 6-10a and 6-10b in Schwalbe’s text illustrate how multitasking can delay task completion. Multitasking also often cause wasted setup time, which increases total project durations.
Critical chain scheduling is based on the assumption that resources do not multitask. This avoids resource conflicts and wasted setup time, which is caused by shifting be- tween multiple tasks over time.
Project buffer is additional time added before the project’s due date. Feeding buffer is additional time added before tasks on the critical chain that are preceded by by non- critical-path tasks. Critical chain scheduling normally results in shorter task estimates than traditional estimates because it removes each individual task’s buffer.
To summarise, critical chain scheduling is a complicated yet powerful tool that involves critical path analysis, resource constraints, and changes in how task estimates are made in terms of buffers.
7.5.4
Program evaluation and review technique (PERT)
Program Evaluation and Review Technique (PERT), yet another project time manage- ment technique, is a network analysis technique used to estimate project duration when there is high degree of uncertainty about the individual activity duration estimates. PERT usesprobabilistic time estimatesthat are based on using optimistic, most likely, and pessimistic estimates of activity durations. PERT is also based on a network di- agram and calculates the weighted average duration estimate of each project activity using the following formula:
Tw =
To+ 4×Tm+Tp
6 , (7.1)
whereTw is the PERT weighted value,To is the optimistic time, Tm is the most likely time, andTp is the pessimistic time.
Do not confuse PERT with network diagrams, which are often referred to as PERT charts.