Project Schedule Model

Project Schedule Analysis and Optimization

Project schedule is not just a Gantt Chart or a timetable that shows what activities are planned to be performed and when. A good schedule is a project schedule model that can answer any “what if” question such as what if an activity is delayed, resource availability changes, material delivery is late, a risk event happens, etc. Project schedule analysis model is the main decision-making tool that shows the expected results of any management decision and provides project management with reliable forecasts for most of the project results.

Project schedule analysis is used for

  • Identifying problems with the schedule network quality such as activities without successors or predecessors, activities whose lags are too long, etc.
  • Finding the best ways to crash project schedule, shortening total project duration.
  • Finding the ways to save money by adjusting activity durations and optimizing resource requirements.

There are different project schedule analysis quality checks developed by different Agencies for their projects. Some of these checks are applicable to any schedule, others are project need- and type-specific.

So, let’s discuss the requirements for a detailed project schedule model for any industry and in any project, and schedule optimization techniques.

1. Model Quality Requirements



1.1. Scope

Project schedule model should cover everything needed to achieve project goals.

1.2. Activities

In the detailed schedules project activities should meet the following requirements.

  • An activity must be performed by a certain resource crew. If some part of an activity requires other resources than another part, then this activity must be divided into two or more activities.
  • Then, activity work volume unit must be the same for every part of an activity.
  • Also, expected resource productivity must be the same throughout an activity execution.
  • Unit cost and material requirements must be the same for any part of an activity.
  • An activity must belong to only one work package.
  • And, activity duration must not exceed two reporting periods (preferably one) or its volume of work must be easily measurable in physical units (meters, tons, etc.) for reliable reporting and forecasting of remaining duration. Exception: Hammock activities that last from one event to another in a project schedule.

1.3. Logic

Firstly, project schedule model must include Start and Finish milestones.

Then, each activity except these milestones and hammock activities must have a predecessor to its start and a successor from its finish.

Starts and finishes of hammock activities are defined by other activities or external events.

If an activity does not have a predecessor to its start, it means that its execution can start before the project start; an activity that does not have a successor from its finish can be finished after the project finish. Both situations indicate that something is wrong with the project logic.

1.4. Leads and Lags

Leads and Lags used in the project dependencies must be defined by technological restrictions and justified.

Volume lags (such as a required distance between working crews) are preferable. Time lags demand special attention because the lag time can pass but the expected volume of work is not completed.

1.5. Constraints

Project schedule model can include No Earlier Than constraints. These constraints must be preferably applied to the schedule milestones that reflect some external events (such as weather constraints, the dates when construction site is available, etc.) in a project schedule analysis.

Hard constraints such as Start On, Finish On, or Finish No Later Than should be avoided because they can contradict the schedule logic. It is only permitted when the scheduling tool permits ignoring these constraints when they are unachievable.

1.6. Resource Assignments

Resources must be assigned to all activities whose durations depend on assigned resource quantities and capabilities.

 

2. Project Schedule Analysis Parameters

2.1. Total Float

PMBOK Guide defines total float as the amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.

This definition is not clear. First, possible delays of activity start and possible extensions of its duration can be different. With outbound dependencies on both activity start and activity finish, possible delays of activity start and activity finish can differ in their duration (see activity flex below).

Second, this definition does not specify what calendar must be used when measuring total float. Usually, it is the activity work time.

If the project schedule analysis was calculated using the Critical Path Method that ignores resource constraints, activity total float is an activity property.

Resource constrained schedules can be calculated in different ways and each project management software uses its own proprietary algorithms. The same activity can have different total floats in different resource constrained schedules for the same project. Therefore, resource constrained total float is an activity property in the current schedule.

With hard constraints (Start On, Finish On, or No Later Than), total float can become negative. This is one of the reasons why hard constraints must be avoided if your software does not prioritize schedule logic over schedule constraints.

2.2. Free Float

PMBOK Guide defines free float as the amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint.

Free Float cannot exceed Total Float for the same activity and, like total float, can depend on the schedule if resource constraints have been considered. In the schedules calculated by CPM ignoring resource constraints both total and free floats are activity properties.

2.3. Critical Path

PMBOK Guide defines Critical Path as the “sequence of activities that represents the longest path through a project, which determines the shortest possible duration.”

Once again, this definition creates questions. Project duration can depend on its last activity with Start or Finish No Earlier Than constraint only. This path is certainly not the longest.

When a schedule does not contain time constraints, Critical Path consists of activities with total floats that are less than a certain time value. This value is zero if project activities and resources have the same calendars.

Critical Path determines the shortest project duration in the current schedule. But this schedule may be improved and shortened. We will discuss this further in this paper in project schedule analysis.

When project resources are constrained, Resource Constrained Critical Path (or Resource Critical Path) can include activities that are not linked to each other.

Resource Constrained Critical Path is the sequence of project activities that determines project duration. As in the schedules that do not have resource constraints, resource critical activities have resource constrained total floats that are less than a certain time value, usually zero if the same calendar is used by all activities and resources.

Unfortunately, most popular project scheduling tools calculate wrong total and free floats in resource constrained schedules and do not show correctly which activities are critical in the schedules with resource constraints.

2.4. Drag

Activity drag is the amount of time that an activity on the critical path is adding to the project duration. Alternatively, it is the maximum amount of time that one can shorten the activity duration before it is no longer on the critical path or before its duration becomes zero

Activity drag is a metric developed by Stephen Devaux.

2.5. Flex

Firstly, Flex is the metric of activity duration flexibility, suggested by Rafael Davila.

Activity Start Flex shows time difference between the earliest possible activity start and planned activity start in the current schedule without violating any constraint and activity finish delay.

Activity Finish Flex shows time difference between the latest activity finish and planned activity finish in the current schedule without violating any constraint and activity start delay.

Both metrics mean increasing activity duration by keeping activity start or finish intact.

2.6. Resource Dependencies

In resource constrained schedules activities are delayed if required resources are busy on other activities. The scheduling software must inform about the reasons for activity delays are due to activity logical dependencies. These reasons can include resource, material supply, space, or financial restrictions that.

Activities are usually become late because required resources are for other activities in project schedule analysis. We may refer to this as resource dependencies, i.e. dummy dependencies that link activities using the resource with those activities that are not on time waiting for these resources. These dependencies must be redefined with each resource constrained scheduling, being the results of current resource constrained schedule analysis.

3. Project Schedule Optimization and Management

Project schedule created using Critical Path Method or resource leveling heuristic can be compressed if activity durations or dependencies may be revised.

3.1 Fast Tracking

The PMBOK Guide defines fast-tracking as a schedule compression technique in which activities or phases normally performed in a sequence are done in parallel for at least a portion of their duration.

Using this technique, the scheduler analyzes whether critical activities may start earlier than in initial schedule, by replacing Finish-to-Start dependencies with Start-to-Start dependencies with lags, or by minimizing lags in Start-to-Start or Finish-to-Finish dependencies.

Activity Drag shows the maximal shortening of project duration by starting an activity earlier than in the initial schedule.

This technique increases the risk of reworks because the succeeding activities start before the works on preceding activities are completed.

3.2. Crashing

Crashing is defined as a technique used to shorten the schedule duration for the lowest incremental cost by adding resources.

Using this technique, the scheduler identifies critical activities that can be completed earlier if additional resources were assigned. It means additional expenses and requires selecting those activities that provide the highest compression with the least cost.

Activity Drag shows the maximum reasonable compression of activity duration.

3.3. Schedule Adjustment

In some schedules, overall project duration can be shortened by increasing the duration of some activities. It does not seem obvious and so we will discuss this method, using a sample project.

The Project “Adjustment” consists of four activities and a Finish milestone (Exhibit 1).

Exhibit 1. Gantt Chart for project “Adjustment”.-min

Exhibit 1. Gantt Chart for project “Adjustment”

One unit of resource A is used on all activities of this project except Activity 2 that requires two units of resource A to be completed in 3 days. Resources are assigned full time.

Next, dependencies between activities of this project are shown in Exhibit 2.

They include two Start-to-Start dependencies with lags of 2 and 1 days and one Finish-to-Finish dependency.

Exhibit 2. Activity Dependencies-min

Exhibit 2. Activity Dependencies

Thus, all activities of this project are critical with zero total and free floats.

Activity 2 has positive start and finish Flexes, which means that it may be started earlier and/or finished later without any impact on the dates of other project activities.

Then, activity Drags are positive for all activities except activity 2, whose drag is negative. Negative drag means that, minimizing this activity duration, we will increase the project duration.

Critical Path Method

The duration of the project schedule created using Critical Path Method is 15 workdays. And it requires four units of resource A, as shown in Exhibit 1. If only three units of resource A are available, resource leveling will make project duration two days longer, as shown in Exhibit 3.

Exhibit 3. Leveled “Adjustment”-min

Exhibit 3. Leveled schedule of project “Adjustment”

 

But what if it is permitted to increase activity 2 duration with the corresponding adjustment of assigned resource workload? Exhibit four shows the resulting schedule if activity 2 duration is increased to 8 days, using the existing start and finish flexes.

Exhibit 4. Adjusted schedule-min

Exhibit 4. Adjusted schedule

Also, the project still requires four resource units but two resource units assigned to activity 2 have only 37.5% workload and are only busy for 3 hours on each of their 8-hour workdays. If two resource units may be replaced with just one with the double workload (6 hours per day), we can get the schedule shown in Exhibit 5.

Exhibit 5. Resource adjusted project scheduling-min

Exhibit 5. Resource adjusted project scheduling

Exhibit 5. Resource adjusted schedule

 

However, increasing activity 2 duration and adjusting resource assignments, we got a 10-days schedule without resource overloads.

So, automatic schedule adjustment and optimization is available for Spider Project users. Users of other scheduling tools can only do it manually. They need to seek such opportunities among the activities with Start-to-Start successors.

3.4. Resource requirements optimization

In our sample project we have optimized resource usage by increasing activity duration and changing the number of assigned resources.

This possibility should be considered for activities with positive start or finish flexes.

Besides, resource constrained schedule can be calculated using different algorithms that provide different results. The software that creates shorter schedules can save a fortune to its users.

You can be interested in a study of resource constrained scheduling capabilities of popular scheduling tools at http://www.planningplanet.com/forums/planning-scheduling-programming-discussion/574396/comparison-resource-levelling-algorithms-di

Creating CPM or resource leveled project schedule model is only the first step towards schedule optimization. Project duration and resource requirements can have better results with these.

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