There is much misconception about the use and interpretation of the S-curves. I find the AACE International Recommended Practice No. 55R-09 is an incomplete document regarding S-Curve Analysis. It worries me that this document is promoting erroneous concepts about the use and interpretation of the S-Curves.

http://www.aacei.org/toc/toc_55R-09.pdf

Among the things the AACE document say is the following:

7. S-Curve Analysis – Cautions:

• It is possible for S-curve information to be misleading instead of instructive. For example, if the project is tracking costs and has pre-ordered several high-ticket items, then the cost S-curve may show excellent progress regardless of the current status of the critical work. In another example, poor productivity will also show a high cost S-curve while schedule progress will be lower than expected.
  • My comment: Actual shows actual progress against time, it does not say about it being excellent or not. It is the projection that tells about the implications of actual progress, we all know no matter if under, above or in between the baseline plan the projection can be under or over.
• Progress S-curves indicate cumulative progress of all work, not just critical work. The project may be behind due to over-emphasis on non-critical work at the expense of the critical work and still show excellent S-curve progress.
  • My comment: If the project is ahead or behind for whatever reasons the current forecast S-curves will tell.
• The S-curve by itself and without any other project information (project status narrative, detail schedule analysis, etc.) may provide a false view of the project. Used in conjunction with other sound project controls practices, S-curves can provide project stakeholders quick and meaningful indications of the overall progress of the project.
  • My comment: The S-curve full set is a complement of the other reports, it represents a summary, a very useful summary if the forecast curves are included. A full set of S-curves might not tell why or how but in no way may provide a false view of the project if it represents true plan. 

We all know s single S-Curve is not enough, we all know there are Early and late S-Curves for the Baseline as well as for the Current Schedule. The Baseline tells us about the frozen initial plan while the Current tells us the history at the left of the Data Date and the Projection at the right of the Data Date.

But the AACE International Recommended Practice No. 55R-09 misses to put attention to the most important components of any S-Curve set. In its analysis the AACE International Recommended Practice No. 55R-09 misses the S-Curve Early and Late projections as if they do not exist even when it shows a single line they call Trend I must assume they are showing some kind of average, though it would be misleading on a true CPM analysis that shows float for remaining work activities. A RP that misses the projected or forecast early and late s-curves is incomplete. If the S-curve information includes the projections it will not be misleading in any way. How come the AACE miss this?

A complete set of S-Curves will not identify the critical path, we all know that, but will tell you if the job is on time or not, is limited on what it disclose but not misleading. The complete set will tell you the overall status without being misleading, it will not tell you why. For decades top management have been looking at these curves for a fast birds-eye view on actual status and projections. For decades software like P3 would give you the complete set at a single click of the mouse. I believe P6 calls the early and late projections remaining early and remaining late. If by omission you start promoting S-curves without showing the projections we will be moving backwards.

The RP misses to mention that the CPM with the available data will project if the finish date will be before, on or after the planned dates. If the projection data is available why not use it for a better understanding of the S-Curves?

The RP Figure 6 shows a single forecast line instead of two, an early and late forecast. It is not like it can only make a forecast if it matches the planned finish, it can make the forecast even if it does not matches planned finish date. The AACE RP does not say how the projected or forecast single line is to be calculated to substitute the early and late forecast lines.

The following is a full set of S-Curves generated from Spider Project software but could have been equally generated by P3. It makes no sense to use current schedule and not consider current projections.

It is ironic that even if you do not compare against a baseline the CPM will be able to generate the current schedule early and late dates [in red in above figure] after data date while a single line is displayed to the left of data date. This is the most relevant set of curves; the AACE RP is misleading by not taking due note of these, the most relevant of S-curves.

The AACE RP does not say how under negative float the S-Curves are to be drawn and interpreted when late bars are projected earlier than early bars. I believe negative float is a wrong manipulation of late dates and that perhaps further manipulation will be required as to make it fit to some common sense logic. I want to see their reasoning.

My call for caution is that it is possible for RP 55R-09 information to be misleading instead of instructive by being so incomplete.