LBMS is typically seen to be something totally different from CPM. That is probably because of the graphics. In fact, LBMS is a combination of an improved CPM algorithm and flowline visualization. (Did you know: a location-based schedule with just one location works exactly like CPM?) The differences and improvements relate to using locations to automate logic (5 layers of logic instead of CPM’s one), using quantities and productivity rates to calculate durations, recognizing that similar work in different locations is related, planning buffers to mitigate risk, and being able to forecast production based on previous trends. These additional calculations do affect the familiar concepts of CPM, namely float and critical path. Because of the fear of unknown, many projects choose to implement parallel processes – running CPM and LBMS side-by-side.
Parallel processes are expensive to manage – large projects may have dedicated full-time people to update the CPM schedule and another team updating the LBMS schedules. When the dates are not in agreement which dates should be used? Subcontractors naturally want to follow the dates which are most beneficial to them. The CPM dates change with every update. The LBMS planned dates never change unless updated by the user. And this creates confusion and people are working from different sets of dates. But the confusion is caused by the new information “stages” of LBMS:
- Target – for performance measurement;
- Current – the current plan to achieve the target;
- Actual – what has actually happened in the past;
- Forecast – what will happen if actual trends continue.
CPM only has the plan (and sometimes one or more saved baseline plans) – therefore, every update results in new dates. Some projects have tried to synchronize their CPM and LBMS monthly by updating the LBMS plan to match with new CPM dates. This loses the value of LBMS – performance comparisons are made with a monthly moving target. Production problems are not noticed in time and everything looks fine until the last few months of the project.
The best option to solve this problem is to use LBMS alone without a parallel CPM schedule. If this is not possible because of political or contractual reasons, a process needs to be defined in advance. In general, LBMS should be used to optimize the schedule because the single layer CPM does not include any optimization capabilities. The LBMS dates and logic will then be matched in the CPM schedule (preferably as an export between two software systems). LBMS is updated at least weekly to provide real-time information for production control. CPM is updated as required by the internal process or contractual requirements (typically monthly or when required by delay analysis requirements). The same progress data should be used in both – LBMS actual start and finish dates should be used to update the CPM. This results in a new plan in CPM. This new plan should not be synchronized with LBMS schedule because it is in fact a forecast. If the LBMS schedule gets delayed by more than a month from planned dates, a recovery schedule should be planned and then the CPM should be adjusted to match this recovery schedule.
Caption: With Vico Schedule Planner and Production Controller, it is easy to view the schedule as a flowline, a gantt chart, and as a resource histogram - all in one screen.
There will be great variances between the LBMS forecasts and the CPM forecasts because LBMS forecasts use production rates. This is natural. The key thing to understand is that CPM measures the actual delay of the critical path on data date while LBMS is trying to mitigate any future delays by giving early warnings of problems and providing tools to calculate control action options.
Parallel processes will always create some issues and conflict, but we have many projects where the two systems have been successfully used together. However, early trials where there was a requirement that CPM and LBMS dates must match at each update ended in failure. By removing this requirement and following the process described above these conflicts can be mitigated and parallel processes can be made to work. This can be an intermediate step towards full implementation until contracts with Owners change to accommodate LBMS’ additional features.
Catch up with the previous articles in this series on LBMS Best Practices:
Part 1: Beyond Start Dates
Part 2: Get the Subs Involved
Part 3: Manpower and Suspensions
Part 4: Control Actions
Part 5: Planning the LBS
Part 6: Clarifying Scope
Also consider working through the Video Training Series for Production Control. The videos contain helpful tips and tricks, as well as a complete overview of LBMS logic and stages.
Having to run parallel processes is just one form of resistance to LBMS. Read previous articles about working together as a team to implement this new way of scheduling:
Resistance to Change in 5D BIM Implementations
When Estimators Block BIM
Comparing Flowline and Gantt Charts Side by Side
You might also be interested in learning more about the differences and similarities between CPM and LBMS. Prepared for the American College of Construction Lawyers 2012 annual meeting, this paper compares and contrasts the traditional critical path method of construction planning with location-based scheduling principles.