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S-Curves and Information Systems in Project Tracking
Project Management · Chapter 5: Project Management Tools and Analytical TechniquesSection 5.7 · lesson 36 of 7312 min · free with a free account
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In this lesson
Tracking cumulative cost against the baseline and the data behind it.
- Build cumulative baseline and actual cost curves and explain why they often form an S shape
- Calculate deviation as actual minus planned and state whether it is measured against spend to date or the whole budget
- Explain why a cost curve alone cannot separate a saving from a delay or a data gap
- Describe how project information systems, dashboards, data quality, and access controls support tracking
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Transcript
Everything said in the video, section by section (1,706 words). Read along, or search it.
Opening Question
Eight Units Below the Line
Fernhollow Regional Bus Company is replacing passenger-information screens at its depots and on its buses, over twelve monthly periods. Devika, the project controller, tracks the money on a cumulative cost curve. At the midpoint, the plan says 46 cost units spent by now. Actual spending is 38. Grant, the sponsor, sees a gap of 8 below the line, and calls it a saving. He wants to release part of it to another project. Is a line below the plan really a saving? The curve alone can't say, and this lesson shows why.
Cumulative Figures
Running Totals Against the Cost Baseline
Start with why curves are cumulative. A reporting period is a fixed slice of time, like a month, for which figures are recorded. One period's figures don't show how much has been spent altogether, or how much should have been spent by now. A cumulative figure is the running total, from the start up to and including a stated period. Because spending adds, a cumulative line never falls, except to correct an error. The plan it's compared against is the cost baseline: the approved budget, spread across the periods in which the money is planned to be spent. The same method works for effort in hours.
Why an S
Slow Start, Build-Up, Peak, and Taper
Plotted against time, many cumulative curves look like a stretched letter S: flat at first, steep in the middle, flat again at the end. Four stages explain it. A slow start: people are assigned, contracts agreed, and few activities can begin, because most depend on earlier ones. A build-up: as early activities finish, the waiting ones open up, more people join, and orders are placed. A peak, with the most activities at once, the largest team, and the heaviest purchases. Then a tapering finish: testing, handing over and closing, with fewer people. The link is slope. A cumulative line's slope in any period equals that period's spending, so spending that rises and then falls draws an S. But the S is typical, not guaranteed. A huge first-month purchase gives a steep start, and a constant team gives something close to a straight line. And it isn't the simulation curve from the last lesson. That one plots probability against possible outcomes. This one plots spending against calendar time.
Worked Example
Building a Ten-Period Baseline Curve
Building the baseline curve is repeated addition. The planned total at the end of a period is the previous total, plus that period's planned spend. Take a ten-period project with a budget of 100 cost units. Planned spending runs 2, 4, 8, 14, 18, then 18, 14, 10, 7 and 5. Running totals: 2, 6, 14, 28, 46, then 64, 78, 88, 95, and 100. The last total must equal the approved budget, which catches any addition error. Periods 1 to 3 spend 14 units, periods 4 to 7 spend 64, and the last three spend 22. Slow, steep, slow: an S.
Actual Curve
Plotting Actuals Up to the Status Date
Actual figures are added as they become known. The status date, normally the end of a period, is the last point to which actual data have been collected and checked. Here the status date is the end of period 6. Actual spending was 2, 3, 7, 11, 16 and 17, so the running totals are 2, 5, 12, 23, 39, and 56. Periods 7 to 10 are shown as not yet incurred, never as zero. A zero would claim nothing was spent, in a period that hasn't happened. The actual line is never drawn past the status date. What might be spent later is a forecast, a separate exercise.
Deviation
Size, History, and Direction of a Gap
A deviation is actual minus planned, at the same point in time. A negative difference means the actual line sits below the baseline. At the end of period 6, that's 56 minus 64: minus 8 cost units. Size: 8 is 12.5 percent of the 64 planned so far, but 8 percent of the whole budget. A report should say which it uses. History: the period differences are 0, minus 1, minus 1, minus 3, minus 2, and minus 1. Together they make the minus 8. A cumulative gap only closes if later periods overspend. Direction: in period 6 the gap grew by only 1, against 3 in period 4, so the lines are nearly parallel. That's a reading of shape, not a forecast. Whether a gap needs action depends on the tolerance, the deviation an organization agreed in advance to accept before escalating. There's no universal number.
Quick Check
Cumulative Position After Month Three
Quick check, on a new project. It budgets 5, 10 and 15 units for its first three months, and pays out 4, 9 and 12. Where does it stand after month 3? Pause, and decide. 5 units under budget. It planned 30 in total and spent 25, so actual minus planned is minus 5. 3 under is only month 3's own difference. The cumulative gap adds every month: 1, 1, and 3.
Key Limitation
Saving, Delay, or Missing Data
Here's the most important limit. A cost curve records money against time, not how much work that money bought. A line below the baseline can mean three different things. The work is on plan and cost less, a real saving. Or the work is behind, so planned activities haven't happened, and their costs haven't arrived yet. Or the money was spent but not yet recorded, a data problem. The curve looks the same in all three cases. Picture two projects, both at 56 against a planned 64. On North, all the scheduled work is done, for less. On South, only part is done. North has a saving, and South a delay. Telling them apart needs a measure of work accomplished beside the money. That's earned value, applied in subchapter 9.5, and its value is itself debated.
Information Systems
Time Entry, Cost Records, and Progress Reports
Where do the actual figures come from? A project information system: the tools, records and routines a project uses to collect, store, combine and report its tracking data. Three streams feed it. Time entry: hours worked, recorded against tasks or cost codes, which give labor cost as hours times an agreed rate. Cost records: purchases, invoices and payments in the finance system. And progress reports: what's complete, what remains, and what's blocking it, from the people doing the work. Each has a typical weakness: hours entered late or from memory, invoices lagging the work, and optimistic completeness near deadlines. The data flow in order. They're entered at source, checked, consolidated into running totals, and reported. A mistake at entry travels unchanged through every later stage.
Dashboards
Few Measures and Meaningful Status Lights
A dashboard is a one-screen summary: a few selected measures, with charts and status lights, so a reader sees the position at a glance. Three choices decide whether it works. Pick a few measures, each tied to a decision the reader really makes. Avoid clutter, since every chart takes attention from the others. And make the status lights meaningful. A red, amber or green light is trustworthy only when its thresholds were agreed and written down. And show the status date, so readers know how old the picture is. What goes on it, for whom, and how often comes from the communication plan, not from whatever the tool shows by default. Summary views are debated. Busy readers act on what they can absorb, but one color can hide a serious problem inside a healthy total.
Data Quality
Timely, Accurate, and From One Source
A curve drawn from poor data is a precise picture of the wrong thing. Three qualities matter most. Timeliness, recording close to the event. Accuracy, matching what happened, with the right amounts, codes and units. And a single agreed source, so two reports can't disagree because they used different copies. Late entry distorts the curve, so a gap at the status date may not be real. Should data be entered centrally, by a trained few, or by each person who did the work? Central entry is consistent. Distributed entry is closer to the event. Which works better depends on the setting.
Integration and Security
Shared Codes and Role-Based Access
Integration connects separate sets of project data, so nobody retypes figures from one tool into the next. A shared identifier, like a work package code carried in the schedule, the timesheet and the cost record, ties effort and cost to the same task. Then a gap can be traced to its part of the project. Information security here means controlling who can view, and who can change, project records. Access is set by role: those who view, those who enter, and those who approve. Keep the roles separate, so the person who enters a cost isn't the one who approves it, and keep an audit trail of who changed what, and when. Legal duties about personal and financial data differ by place and sector, so follow the organization's policies, and consult counsel. This is management guidance, not legal advice.
Quick Check
A Late-Entered Invoice
Quick check. A large delivery arrives in period 7, but its invoice isn't entered until period 9. What does the actual cost curve show for periods 7 to 9? Pause, and decide. Too low, then a jump. Periods 7 and 8 understate the real spending, and period 9 catches up all at once. It isn't a true dip, since the money was committed. And nothing pushes the curve above real spending. Any gap read in period 8 would be false.
Back to Fernhollow
Checking Before Calling It a Saving
Back to Devika and Grant. Her curve supports at least three accounts of the gap of 8. Supplier prices came in low, a real saving. Installation is behind, so purchases haven't happened yet. Or invoices arrived but weren't entered, so the actual is understated. A saving can be released. A delay means the money will still be needed. And unrecorded invoices mean the figure shouldn't be used yet. The steering meeting is in six days. She asks the work package leads what share of each installation is done. She asks finance for unentered orders and invoices, and compares contract prices with the baseline. She won't call it a saving until those are in. Grant gets a qualified message, and the release waits one cycle. An avoidable error costs more credibility than a delayed decision.
Key Takeaways
Curves, Deviations, Limits, and Data
To recap. A cumulative curve adds each period's figure to the running total, and often bends into an S. Deviation is actual minus planned, with its base stated. A cost curve alone can't tell a saving from a delay, because it doesn't show work done. And the curve is only as good as its data: timely, accurate, from one agreed source.
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