PART 3 | Why Use S-curves? | S-curve Analysis
Generating S-curves | Conclusion

In the last Part 3 of this paper, I provided a worked example of an S‑curve from a real project. In this Part 4 I shall present further justification as to why S‑curves should be used and analyzed.

Why Use S-curves?

S-curves are an important Project Management Tool. They allow the progress of a Project to be tracked visually over time, and form a historical record of what has happened to date. Analysis of S‑curves allows Project Managers to quickly identify:

• Project Progress
• Project Growth or Contraction (Man Hours and/or Cost, Duration)
• Project Start and/or Finish Slippage

Although Gantt Charts also display Project information visually, they can quickly become cumbersome to work with, especially with Projects having hundreds or even thousands of Tasks.

Determining Project Progress

Comparison of Target and Actual S-urves reveal the progress of the Project over time. In most cases, the Actual S‑curve will sit below the Target S‑curve for the majority of the Project, due to many factors, including under reporting of accurate Task % Complete values, etc. Only towards the end of the Project will the curves converge and finally meet. Although the Actual S‑curve may sit above the Target S‑curve for a period during the Project, it can never finish above the Target S‑curve at Project completion.

Figure 22: Calculating Project Progress using S-curves

Analysis of the above S-curves reveals the Project as of the 4th of February 2016:

• Is 11.05 % (51.05% – 40.00%) behind target
• Has been behind target since it commenced

Ahead of Schedule?

If the Actual S-curve sits above the Target S‑curve, the Production Schedule should be examined to determine if the Project is truly ahead of schedule, or if the Production Schedule contains unrealistic or incorrect % Complete values for ongoing Tasks.

The Target S-curve calculations assume Man Hours and/or Cost will be distributed evenly for each Day of the Task. For example, a Task with a 5 Day Duration and 100 Man Hours will be assumed to be completed at a rate of 20 Man Hours per Day, as shown below.

Table 13: Task Target S‑curve Data (Showing Man Hours Distributed Evenly per Day)

If a Task has the majority of the Man Hours and/or Cost completed at the beginning, in the middle, or at the end of the Task Duration, the Actual S‑curve may be pushed higher than the Target S‑curve (depending on other Tasks ongoing within the same time period).

The planned Task in the following Table has the majority of the Man Hours scheduled to be completed at the beginning of the Task.

Table 14: Planned Task Details

Using a Cut Off Date set to the 4th of February 2016, and a Task % Complete value of 75%, the Actual S‑curve calculations for the planned Task are shown below.

Actual S‑curve Date	Cut Off Date: 4th Feb 2016
Date	02‑Feb‑2016	03‑Feb‑2016	04‑Feb‑2016
Totals Per Day	25	25	25
YTD Totals	25	50	75
% Complete	25%	50%	75%

Table 15: Task Actual S‑curve Data

Figure 23 illustrates how such a Task can result in the Actual S‑curve sitting above the Target S‑curve.

Figure 23: Actual and Target S-curves (Actual S-curve sits above Target S-curve)

The accuracy of the Production Schedule and any S‑curves generated from it may be improved in this instance by splitting the planned Task into two Tasks.

The original Task should be revised to have a Duration of 2 Days, 75 Man Hours, and be set to 100% Complete.

Table 16: Revised Task Details

A new Task with a Duration of 2 Days, 25 Man Hours, and set at 0% Complete should be added.

Table 17: New Task Details

Determining Project Growth or Contraction

Comparison of the Baseline and Target S‑curves quickly reveals if the Project has grown (Target S‑curve finishes above Baseline S‑curve) or contracted (Target S‑curve finishes below Baseline S‑curve) in scope.

A change in the Project's scopes implies a re-allocation of resources (increase or decrease), and the very possible requirement to raise contract variations. If the resources are fixed, then the duration of the Project will increase (finish later) or decrease (finish earlier), possibly leading to the need to submit an extension of time claim if the Project will finish late.

Figure 24: Calculating Project Growth using S‑curves

Analysis of the above S-curves reveals the Project as of the 4th of February 2016 has grown:

• in Man Hours by 5.56% (105.56 – 100.00)
• in Duration by 1 Day (6 Days – 5 Days)

Determining Slippage

Slippage is defined as:

"The amount of time a Task has been delayed from its original Baseline Schedule. The Slippage is the difference between the scheduled Start Date or Finish Date for a Task and the Baseline Start Date or Finish Date. Slippage can occur when a Baseline Schedule is set and the actual dates subsequently entered for Tasks are later than the Baseline dates or the actual durations are longer than the Baseline Schedule durations".[2]

There are two types of slippage: Start Slippage, and Finish Slippage.

Start Slippage

Start Slippage occurs when the estimated / actual Start Date of the Project / Task is later than the Baseline Start Date. In other words, the Project / Task has started later than planned. If the Project has already commenced, nothing can be done to eliminate or reduce the Project's start slippage. An Extension Of Time Claim and/or Variation may need to be submitted depending on the reasons why the Project started later than planned.

Figure 25: Calculating Project Start Slippage

Analysis of the above S‑curves reveals that as of the 4th of February 2016, the Project has:

• Started 1 Day late

Finish Slippage

Finish Slippage occurs when the estimated / actual Finish Date of the Project / Task is later than the Baseline Finish Date. In other words, the Project / Task will finish / has finished later than planned, either because the Project / Task started late, or the Project / Task Duration has increased. Obviously if a Project / Task experiences Start Slippage, Finish Slippage will also occur if the Project / Task Duration remains the same. Additional resources will need to be allocated or additional hours worked in order to eliminate (or at least reduce) the Project's Finish Slippage. An Extension Of Time Claim may need to be submitted if the Finish Slippage cannot be eliminated or reduced to an acceptable level.

Comparison of the Baseline S-curve and Target S‑curve quickly reveals if Project Start and/or Finish Slippage has occurred. (I.e. the Target S‑curve starts and/or finishes to the right of the Baseline S‑curve).

Figure 26: Calculating Project Finish Slippage

Analysis of the above S‑curves reveals that as of the 4th of February 2016, the Project will:

• Finish 2 Days late

PART 3

S-curve Analysis

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