Job Shop Scheduling- CPM or ERP?


CPM (Critical Path Method) is ideal for multi-project scheduling in environments characterized by scarce resources.

CPM works best for the management of work where there are a small fixed number of project templates, where there are few branching decision points and where tasks can be given durations at the template level.

Given a CPM network diagram, with task resource requirements and task durations, a whole world of forward scheduling, resource allocation, leveling and balancing opens up, complete with performance measurement facilities and predictive capabilities.

It all starts with ES-LS-EF-LF calculations (Early Start, Late Start, Early Finish, Late Finish). If you take the difference between LS and ES at any task you have “float”. So long as you start a task no later than its LS date, your project will finish on time. But, the effects are cumulative.

It has been our experience that if you see an erosion of float during the 1st 20 percent of a project, you are not likely to reach project milestones without an extraordinary expenditure of effort and cost.

This is because most work performance is characterized by “S curves” where initial efforts result in little progress toward objectives. Once “liftoff” is achieved projects typically move forward smartly up to about the 90% state (i.e. 90 % extent of achievement of objectives, not time), after which progress toward objectives slows down. Hence, the term, “S curve”.  It’s possible in some situations to be 90% complete with ½ of time remaining.

Traditional CPM was like BPM.  The main focus of early implementations was on the planning side. For performance assessment/measurement purposes, the ES-LS-EF-LF date set needs to be augmented by actual start/actual finish dates (AS-AF dates) so that network templates can be improved for future use based on the analysis of run-time data. Same for costs, same for resource pools.

Now, re the possible use of CPM for job-shop fabrication, it’s my view that BPM/ERP can do a better job providing you can export your network to a CPM environment that is capable of performing ES-LS-EF-LF calculations and you are able to post the results of such calculations back to your BPMs.  The other option is to look for a BPMs that has CPM capabilities.

The bad news is there do not seem to be many BPM/ERP environments in the marketplace and even less  BPM/CPM/ERP environments.

About kwkeirstead@civerex.com

Management consultant and process control engineer (MSc EE) with a focus on bridging the gap between operations and strategy in the areas of critical infrastructure protection, connect-the-dots law enforcement investigations, healthcare services delivery, job shop manufacturing and b2b/b2c/b2d transactions. (C) 2010-2017 Karl Walter Keirstead, P. Eng. All rights reserved. The opinions expressed here are those of the author, and are not connected with Jay-Kell Technologies Inc, Civerex Systems Inc. (Canada), Civerex Systems Inc. (USA) or CvX Productions.
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