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Why so many poor schedules?

Why so many poor schedules?

Schedule logic is not the issue, what makes our schedule models so poor is how we deal with resources. Without resources nothing is done.  Schedules that do not take into account all resource constraints are incomplete and most probably wrong.

We favor simplicity over substance and give the project manager very poor schedules he must manually adjust as soon as he realized the plan is no good.

1.     The project manager understands the difference between effort and quantity and the need for partial workloads.  He will not plan the impossible while most software will allow the impossible under partial workloads.

2.     No project manager is to delay an activity because his model delayed the start of the activity when out of 10 resources only 8 are available and it is feasible to work at a reduced pace with 8.  He knows he will finish the activity earlier the sooner it starts.  

3.     EVM jargon is not used at the jobsite. Site people and project managers manage the schedule using a common language; productivity and volume of work is spoken every day.  Schedules that hide what are the planned production rates are difficult to manage.

4.     A project manager knows that if resources are consumed but not replenished the activities are delayed until more become available. He knows the difference between renewable and consumable resources.  He understands people are not consumed by the activities but used temporarily to be released as soon as the activity no longer uses them; he knows these are renewable resources. He knows consumable resources are not automatically replenished as renewable resources do, he realizes the schedule must consider their availability at all times if to be feasible.

5.     A project manager knows that without funds the project is stopped.

6.     A construction project manager knows he shall not move out elevated slab forms from a building before the last pour have been placed and cured.  He might not know that the scheduling jargon for such resources is spatial resources but he knows how to deal with it.

If the scheduler do not know the difference between effort and quantity, do not understand the need for partial workloads, do not realize the need for variable resource quantity and workloads for efficient scheduling, do not realize the existence of consumable resources, do not realize the need to have financial resources in place and what spatial resources are the chances he will deliver a poor schedule are high.

Unless the scheduler makes sure all resource constraints are adequately handled the project manager will have no other option than to improvise on the field.

For a more detailed presentation of these concepts and how already commercially available software can efficiently deal with all (and more) please refer to the following link:

Thank you for your attention.

Rafael Davila


The following is another

The following is another example that illustrates skill replacement, all required information is available.

There are four independent activities that are not linked to each other. There are 3 resources that can work on the activities, each with different production rates as shown, only one will be assigned to each activity. Each activity volume of work is 80 units. The activity duration is a function of resource production rate and the volume of work. The schedule is not resource leveled. There is a common calendar of five eight hours days a week.

1. What is your best feasible (resource leveled) schedule?

2. What is your methodology?

3. What would be your solution if using the resource leveling engine within your model?

4. What would be your solution if activity 1 volume of work is increased to 100 units?

These are basic calculations a scheduler shall be able to model so that if there is a delay or change in resource availability the model will automatically make a good selection.

Most of the problems are born

Most of the problems are born because of the scheduler not being able to model beyond elementary activity linking.

When variable workloads are

When variable workloads are necessary many get lost. Such is the case in design jobs where designers are assigned to several activities during same day but different hours each day. Please take a look at the following link for an image of such scenario.


  • Under this scenario many schedulers will abandon the idea to resource level the resources using the CPM resource leveling engine and will resort to manual scheduling. This approach is inefficient, in most cases will result in substandard schedules.

Another example of poor

Another example of poor schedules happens when the scheduler does not understand how to model partial workload assignments.

Take a look at the following schedule for a multistory concrete structure. I consist of seven activities and span a total duration of 133 days. The crane is a resource that is shared bust must be leveled as not to overload the resource.


  • If assigning some activities to different shifts and using partial workloads there is no need for additional activities to provide enough information to prevent overloading the crane and to manage the resource.
  • Under this scenario schedulers that do not understand shift-work and partial workload models will abandon the idea to level the resource and pray for the best.
  • Some will create a hammock and assign the crane to it, but this will not level the resource.

Another typical example of

Another typical example of poor scheduling is when modeling shop scheduling the scheduler uses predecessor logic to force the resource leveling or when using standard renewable resource.

- If using predecessor logic every time a job is delayed for whatever reason all soft dependencies must be realigned in order to get a feasible schedule. This usually results in poor realignment that is far from optimal.

- If using renewable resources the resources might leapfrog in unintended ways leading to poor schedules, at times unfeasible. The shown links that spans several jobs are resource dependencies created by the software, in no way are hard links.

Spatial resources are required by a group of activities, rather than a single activity as renewable resources. The spatial resource is occupied from the first moment an activity from the group starts until the finish of all activities from that group. They can be modeled if using consumable resources.

Another example of why there

Another example of why there are so many poor schedules due to basic calculations is that few schedulers consider in their models financial constraints.

Financial constraints are a type of consumable resource that must be leveled in order to avoid disruption of your portfolio. Cash flow is unknown before resource leveling; a good model shall calculate cash flow based on how activity moves, incomes (supply) as well as expenses (consumption) are not fixed with time. The model shall be able to replicate real world scenarios.

For example, activities create the need to rent a piece of equipment as well as its release, they define the rental period as activities move. This piece of equipment pays rent no matter if idle or not during the rental period, but when in use it will consume some materials that have some costs.

The resource leveling shall consider all consumable, renewable and financial resources as they happen. The following is an overly simplified model for illustration purposes; we know the devil is in the details.

Another example of why there

Another example of why there are so many poor schedules due to basic calculations is that few schedulers consider in their models that work production rate might vary as its gets cold/hot. You can try yourself a simple scenario where the crew production rate will depend on the season the activity moves.

Another example of why there

Another example of why there are so many poor schedules due to basic calculations is that few schedulers consider that efficient resource assignments are not always fixed.

If out of ten scheduled bricklayers only nine are available no rookie PM is going to delay for weeks an activity until the missing becomes available. He is not to let idle the nine available, he is not as dumb as no to start the activity with fewer resources at a slower peace until more become available. He knows that in this way the activity and its successor will finish earlier if he does so.

Still some schedulers do not take this into consideration making their plans unrealistic and leaving the PM with no other option than to improvise. If this happens frequently the schedule is useless.

But planning for variable quantity is so easy. For illustration purposes in the following example there are 19 resources A available, as well as 19 resources B available.

Every time the schedule moves and things do not happen as planned the availability changes and so the activity durations.  If the scheduler makes a good model at a single click of the mouse he should get a resource leveled schedule. In a job with hundreds or thousands of activities, each competing for a limited amount of resources it is not practical to manually level the schedule, in Monte Carlo runs it is impossible. 

As an example of how bad some

As an example of how bad some schedulers can be at making reliable plans we can look at the following scenario. As soon as bricks become depleted activities will be stopped until more are available. Consumable resources are just one of the many resource types a scheduler must consider.

Can you provide a good reslurce leveling solution to this scenario?

For a scheduler it shall be easy as it is so simple, only 5 installation activities, four deliveries and only 3 consumable resources.

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