Progress S-curves (output)
As might be expected, the foregoing factors have a considerable impact on total
production especially as represented by the more familiar output or progress
S-curves.
A complete determination of the project status and projections to final completion
for management action can perhaps best be tracked by an integrated cost/schedule
system or technique known as "Earned Value and Performance Measurement"
(Kerzner 1989). The earned value, i.e. the Budgeted Cost of Work Performed (BCWP),
is determined at regular intervals during the course of the project. At the same
time, the Actual Cost of Work Performed (ACWP) is also determined, and both are
compared to the baseline plan which is the Budgeted Cost of Work Scheduled (BCWS).
By presenting these results graphically as S-curves, the variances in cost and
schedule can readily be seen, and by analyzing the results relative to the baseline
plan S-curve, estimates can be made of anticipated variations at completion.
The key elements of the technique are shown in Figure 3.
Figure 3: Earned value and performance measurement
The technique is especially useful on projects involving a large number of
significant activities by different trades and/or under conditions which change
during the course of the work. There are, however, several weaknesses in the
approach (Meredith 1985). Cost data must be collected which reflects the actual
progress of the work, work in progress must be measurable and it must be measured.
Consequently, this form of tracking requires significant additional effort or
qualified dedicated staff to collect reasonably reliable data. This is particularly
true where large purchases of off-site equipment may involve staged payment assessments.
Since the results are at best estimates of work-in-hand and the final results
are estimated projections, the technique is not usually considered worth the
effort on most projects. The exceptions are large complex projects, or projects
on which this approach is required under the terms of the contract. When the
technique is adopted, an essential element in its successful use is a realistically
shaped baseline plan S-curve.
A better strategy for tracking progress is to identify the major critical activities
on site that are measurable and plot those S-curves as surrogates for the whole
job or stages of the job. Figure 4 shows three S-curves[2]
illustrating different major activities. To facilitate comparison they are shown
plotted as percent progress against percent time.
Figure 4: Three examples of progress S-curves
Curve (a) shows progress on a 5560 pipe pile driving contract lasting 137 working
days. Curve (b) shows the cumulative progress on a 180,000 cu. yd. bulk excavation
contract lasting 15 weeks. Notice the progressive addition of plant as the work
opens up in the beginning, and the subsequent removal of plant as the availability
of work runs out towards the end. Curve (c) shows progress on a 7-month civil
contract as reflected by the approved monthly measurement progress billings.
Figure 5 shows measured progress on a 42,000 cu. yd. structural
concrete activity of 16-months duration in Ontario, Canada. This curve is interesting
because it clearly shows the slow down in progress over the winter months. The
original data indicates that the virtual cessation of activities due to cold
weather was only two-and-a-half months. However, due to the S-curve effects just
before and after the cold weather cessation, the total impact of this condition
was closer to three-and-a-half months. In the preparation of the original construction
schedule, this situation could have been reasonably foreseen and an appropriate
adjustment made to the "standard" S-curve profile.
Figure 5 - Placing of formed structural concrete.
Total concrete = 31 900 m3; total time = 16.5 months
2.
From the author's personal records of progress tracking.
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