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Henry Gantt's Legacy: Gantt Charts and Bar Graphs
Project Management · Chapter 5: Project Management Tools and Analytical TechniquesSection 5.4 · lesson 33 of 7311 min · free with a free account
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In this lesson
Building and reading Gantt charts, and knowing what they hide.
- Identify the parts of a Gantt chart, including task bars, milestones, summary bars, baselines, and the status line
- Place bars using forward-pass start and finish positions rather than by eye
- Read progress at a status date by comparing reported completion with elapsed time over duration
- Explain what a baseline gap does not reveal and the limits of Gantt charts compared with network diagrams
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Transcript
Everything said in the video, section by section (1,560 words). Read along, or search it.
Opening Question
Is a Diamond on the Chart a Promise
Nadia, a project coordinator, shows Peter a one-page chart for refurbishing the Ashgrove branch library. A diamond marks the reopening, at the end of week 12. Peter studies it and says: then we open in week 13. I'll tell the council. He read the chart as a promise. Is it one? What does a bar chart really say, and what does it hide?
Definition
Tasks as Bars Against a Time Scale
A Gantt chart is a schedule drawn as a bar chart. Each task gets its own row, and each task is a horizontal bar, placed against a time scale. The bar's position shows when the task is planned to happen, and its length shows how long it's planned to take. Bar graph scheduling is the wider term, for any method that draws tasks as bars against time. The Gantt chart is its best-known form. The name comes from Henry Gantt, an American engineer who developed scheduling charts in the early twentieth century. Others devised bar methods too, such as Karol Adamiecki's harmonogram. Who came first is a question for historians, and this lesson takes no position.
Chart Parts
Task List, Time Axis, Bars, and Milestones
Every Gantt chart is built from a small set of parts. The task list runs down the left, one task per row. The time axis runs across the top, in equal units such as weeks. A task bar's left end is the planned start, its right end the planned finish, and its length the planned duration. A milestone is a zero-duration marker, usually a diamond, for an event or decision, like a design approval. It takes no time, so it has no length. And a summary bar spans a group of related tasks, from the earliest start to the latest finish, so a whole phase reads at a glance.
More Chart Parts
Links, Progress Fill, Baseline, and Status Line
Four more parts carry logic, progress and comparison. A dependency link is an arrow from one bar to another, showing that one task's timing depends on the other. A progress bar is a darker fill inside a task bar, from its left end, showing the share reported complete. A baseline is the approved schedule, saved so later versions can be compared with it. A baseline bar is a thin bar beneath the task bar. And the status date is the date the chart is current to, drawn as a vertical line across every row. Read them together. The task bar says what's planned, the baseline what was planned before, the progress fill what's been reported, and the status line when.
Position Convention
Week n Spans n Minus 1 to n
Here, positions are counted in weeks since the project began. Week n runs from position n minus 1 to position n. Finish position equals start position plus duration. A task that begins at the start of week 3 has a start position of 2. With a duration of 2 weeks, it finishes at position 4, the end of week 4. The bar runs from 2 to 4. Choose a scale to suit the project. A fine scale gives precision but width. A coarse one gives a readable overview, but hides short tasks and small delays. And check whether the axis counts every calendar day, or only working days. The same task sits in different places under each.
Worked Example
Eastmere Hall Kitchen Refit Tasks
Now build one: a kitchen refit at the Eastmere Community Hall. Start from the work breakdown structure, so every bar traces to agreed scope. Order the tasks by planned start, keeping related tasks together. Then set the durations, in weeks. Site assessment, 2 weeks. Design drawings, 2, after the assessment, then the milestone, design approved. Order materials, 1 week, after the approval. Strip out the old kitchen, 2 weeks, after the assessment. Install fittings, 3 weeks, after both the materials and the strip-out. Ventilation, 2 weeks, after the strip-out. Inspection, 2, after fittings and ventilation. Commissioning, 2, after inspection, and the milestone: kitchen reopens.
Placing Bars
Forward Pass Sets Each Bar
Don't place bars by eye. The forward pass gives each task its earliest start and finish. Site assessment runs from 0 to 2. Drawings run from 2 to 4, and the design approval sits at 4. Materials run from 4 to 5. Strip-out follows the assessment, from 2 to 4. Fittings wait for the later of materials, at 5, and strip-out, at 4. So they run from 5 to 8. Ventilation runs from 4 to 6. Inspection waits for the later of 8 and 6, so it runs from 8 to 10. Commissioning runs from 10 to 12, and the kitchen reopens at the end of week 12, set by the chain 1, 2, 3, 5, 7, 8.
Quick Check
A Task Following Finishes at 7 and 10
Quick check. A new task follows two others, which finish at positions 7 and 10. It takes 3 weeks. Where does its bar run: from 7 to 10, from 10 to 13, or from 17 to 20? Pause, and decide. From 10 to 13. The task waits for the later finish, 10, then adds its 3 weeks. Starting at 7 would begin before one predecessor is done. And finishes are never added together.
Status Date
A Longer Strip-Out at Week 6
Now the chart at a status date, the end of week 6. Tasks 1, 2 and 3 finished on their baseline dates. The strip-out took 3 weeks, not 2. Its bar now ends at 5, while its thin baseline bar still ends at 4. Fittings still start at 5, because materials finish at 5 anyway. Ventilation follows the strip-out, so it slides a week, to run from 5 to 7. Inspection still starts at 8, so the finish stays at week 12. Two tasks cross the status line. Fittings report 40 percent complete, and ventilation 25 percent.
Reading Progress
Elapsed Time Divided by Duration
Read status from the line. A bar wholly left of it should be complete, and a bar wholly right of it shouldn't have started. For a bar that crosses it, the expected share is the time elapsed since its start, divided by its duration. Fittings started at 5 and last 3 weeks. One week has elapsed, so a third is expected. It reports 40 percent, about 7 points ahead. Ventilation started at 5 and lasts 2. Half is expected, and it reports 25 percent, 25 points behind its bar, and further behind the baseline. This is a reading aid, not a measurement. It assumes the work is spread evenly, and someone still has to ask who reported each percentage, and how.
Quick Check
Sixty Percent at Position Seven
Quick check. The status date is position 7. A task runs from 6 to 10, and reports 60 percent complete. Is it ahead of its bar, on its bar, or behind it? Pause, and decide. Ahead. One of its 4 weeks has elapsed, so a quarter is expected, and 60 percent is well beyond that. If the work really is front-loaded or back-loaded, that reading could mislead, so ask how the figure was made.
Baseline Overlay
A Gap Shows Difference, Not Cause
The baseline overlay puts the approved plan beside the current one, so differences show. But a gap answers none of three questions. Cause: was the task estimated too optimistically, started late, grown in scope, or short of people? Or was the baseline unrealistic from the start? Effect: the strip-out's extra week didn't move fittings at all, but it moved ventilation, which still finishes a week before inspection begins. That spare week is ventilation's float, how late a task can be without delaying its successors or the finish. Float is calculated in subchapter 9.6. And standing: if the baseline is replaced every time the project slips, the gap vanishes and the history is lost. Record every replacement, with its reason.
Limitations
Hidden Links and No Uncertainty
Compared with a network diagram, a Gantt chart shows time by position, which a network can't. But three limits deserve emphasis. First, dependencies are hard to see on a crowded chart, so many charts draw only some links, or none. Second, logic can be hidden. A bar sits where it does because of a link the reader can't see, so someone may move it without knowing what depends on it. Third, a bar shows no uncertainty. It has one length, and says nothing about whether 2 weeks or 5 are plausible. Which should be the master schedule document is contested. One side says most readers follow a chart easily. The other says the logic is the schedule, so the chart should only be a report drawn from it.
Back to Ashgrove
A Date With a Stated Condition
Back to Nadia and Peter. Her chart showed the order of work, the closed weeks, who was needed when, and the reopening milestone. It hid three things. The carpet estimate came from one supplier quote, with no range. Two links, between the wiring and the shelving, weren't drawn. And contractors would report progress their own way. Peter wanted one page and one date for the council. So Nadia kept one page, added baseline bars and a status line, and attached a short note. The note says the reopening is a planned date, to be confirmed at the inspection milestone. It names the carpet as the least certain estimate, and lists the undrawn links. Peter got a date with a stated condition, not a clean promise. A visibly conditional date costs less than an announced date, later withdrawn.
Key Takeaways
Position, Length, Status, and Baseline
To recap. A bar's position shows when, and its length shows how long. Place bars with the forward pass, not by eye. Read status at the status date line: elapsed over duration, as a reading aid. A baseline gap shows a difference, not its cause. And a chart can hide links and uncertainty, so a date on it is a plan, not a promise.
About Project Management at UPI Study
Project Management (Business 230) is a self-paced online course of 73 video lessons in 12 chapters, from starting and defining a project to planning, budgets, scheduling tools, proposals, contracts, execution, monitoring, closure, teams and certification. It carries a NCCRS credit recommendation for 3 semester credits.
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About Henry Gantt's Legacy: Gantt Charts and Bar Graphs
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