The Gantt Chart Hardly Needs an Introduction
September 26, 2019
“Human history teaches us, however, that economic growth springs from better recipes, not just from more cooking.”
–Romer, Paul M. “Economic Growth.” In the Concise Encyclopedia of Economics, 2008
In the 1965 published book, “CPM in Construction Management”, the author, J. O’Brien, challenges the notion of a new visual planning aid. If the bar graph is so well suited to construction activity, why look for another planning aid? The reason lies in the fact that the bar graph is limited in what it can present. In the preparation of the bar chart, the scheduler is almost necessarily influenced by desired finish dates, often actually working backward from the completion dates. The resultant mixture of planning and scheduling is often no better than wishful thinking.
If a bar graph is carefully prepared, the scheduler goes through the same thinking process that the CPM planner does. However, the bar graph cannot show (or record) the dependent tasks which control the project progress. Later, even the originator is often hard-pressed to explain his plan using the bar graph.
Nowadays, the Gantt Chart has many supporters. Gantt chart has evolved to become the primary means of communicating project plans and for good reason. The Gantt chart is the primary visualization method in many of the project scheduling software options available today. Gantt chart software was created to automate the planning and scheduling process to create task dependencies, add milestones, or identify the project’s critical path.
But then a Gantt Devotee Broke Rank. In 2012, Dave Garrett, founder of the online community ganttheads.com sent an email to his network with the subject of “Subject: Project Management is Changing (and so are we…)”. The well-known project manager recognized the coming changes saying “…times change and many project leaders are much more focused on their KanBan Boards than on a linear schedule.” He changed the online community name to Project Management, noting changes to tools and approaches.
“…times change and many project leaders are much more focused on their KanBan Board than a linear schedule.”
Mid-1910s Henry Gantt creates Gantt charts as a tool for production scheduling in the job shop environment
1931 Construction begins on the Hoover Dam using Gantt charts
1956 The Interstate Highway System begins construction; Gantt charts are used as the scheduling method
1957 CPM improves on Gantt charts by expressly recognizing the logic ties or task dependencies 1957 to Mid-1980s Gantt charts survive CPM as one of the formats often used to display CPM schedules
Mid-1980s Gantt charts with logic ties aka Logic Gantt charts become the CPM software graph of choice
2008 Dr. Gui introduces the graphical path method or GPM® at the PMI College of Scheduling Annual Conference in Chicago
2012 Patrick Weaver debunks the notion that Bar Graphs were invented by Gantt
Many project superintendents will use an Excel Gantt chart to create a project schedule. However, Excel is not an effective tool to create a Gantt chart for many reasons. A simple Gantt chart does not have the ability to communicate, track and analyze project data. For example, the project schedule will not automatically update in real-time like it would in other scheduling tools. In fact, it is nearly impossible to track changes and alter timelines within an Excel document. Most importantly there is a breakdown in logic and you cannot involve resources in the project schedule when using an Excel Gantt Chart.
Original network notation introduced with CPM in 1957; arrows are used to denote activities, and finish-to-start (FS) dependencies are modeled by connecting the finish nodes of activities to their respective successors’ start nodes.
Network notation introduced in 1961 and extended in 1963; boxes or scaled task bars (shown) are used to denote activities, and activities are connected with logic ties to model FS dependencies and PDM dependencies known as start-to-start (SS), finish-to-finish (FF) & start-to-finish (SF)
Time-scaled network notation introduced with GPM in 2008; dimensioned bars with end nodes are used to denote activities, and activities are connected with multiple-arrow links through their end nodes or embedded nodes to model FS dependencies or PDM dependencies, respectively.
Learn more about the Graphical Path Method.
The LDM Approach to PDM Logic
A 10-day SS logic tie between Frame Walls and Rough-In MEP is conveyed by connecting an embed offset 10 days after the start (tail) node of Frame Walls with a vertical (V) link (in this case) to the successor’s start (tail) node
10 days remaining in Rough-In MEP after Frame Walls finishes is conveyed by connecting the finish (head) node of Frame Walls with a horizontal-vertical (HV) link (in this case) to an embed offset 10 days before the finish (head) node of Rough-In MEP
Planners Can Drive Activity Dates After Early Dates Without Using Start Constraints
If Planned Start Date > Early Start Date, Activity Has Drift (May Revert to Earlier Dates)
Drift + Float = Total Float
Logic Tie Float aka Link Gap
Total Floats Left of the Data Date
Because GPM calculates total floats as of 31 Jan 13 both before and beyond the data date, GPM algorithmically identifies the critical path left of the data date
Data date (DD), the update closing date, splits the updated schedule between the progressed (as-built) portion left of the DD and the forecast portion right of the DD
Total Floats Left of the DD May Change as Total Floats Right of the DD Change
As total floats and the critical path change right of the data date, so do correspondingly change forensic total floats, and the as-built critical path left of the data date is the critical path existing for that update
Because GPM upholds the then-existing critical path principle, collapsing the as-built schedule in segments yields a solution that is symmetric with the retrospective time impact analysis counterpart
Context is the Key to Resource Leveling
Planners can Pull the Schedule and Build the Network in Reverse Order (Backward from Completion)
|GPM||CPM with Logic Gantt Charts|
|Time-scaled LDM networks innately convey PDM logic||Gantt charts with logic ties fail to convey logical flow|
|Stakeholders, not the algorithm, drive activity dates||Reliance on CPM algorithms biases activities to early dates|
|Constraints are not needed to start activities on planned dates||Constraints are required to start activities on planned dates|
|GPM planned dates do not supersede early dates||SNE* constraints supersede CPM early dates, forfeiting drift|
|If planned start > early start and < late start, drift and float exist||Drift is always 0; If early start < late start, total float exists|
|Link gap measures logic tie leeway (different from free float)||Logic tie leeway is not a concept in CPM|
|SNE constraints are reserved for contractually-imposed dates||Does not distinguish planned dates from constrained dates|
|The schedule can be generated either forward or backward, or both||The schedule can only be generated forward from the project start event|
|Total floats are algorithmically calculated left of the data date||Total floats are only available forward of the data date|
|The as-built critical path is calculated left of the data date||As-built critical path cannot be calculated by CPM alone|
|Stakeholder strategies in context are key in resource leveling||Automated software heuristics drive resource leveling|
|Capable of modeling floating & pacing risks in simulation||Early dates bias does not allow modeling of floating or pacing risks|