Resources, Effort, and Duration

The amount of available resources, whether labor or nonlabor, affects estimates of work and its duration, as well as resources available for subsequent activities. Labor is, of course, a human resource; nonlabor resources can be subcontracts, consumable material, machines, and other purchased equipment. Labor and equipment are measured in units of time such as hours and days; material resources are measured in cubic feet, tons, pallets, or the like. Resources can depend on time—generally labor but also equipment or space rented per period—meaning that they increase in cost as time runs longer. Alternatively, they can be time-independent in the sense that they cost the same regardless of time—for example, material costs. Some activities use both labor and nonlabor resources.

Labor Resources

In general, the amount of work, in person-days, required to complete an activity is equal to the duration of the activity multiplied by the number of labor resource units assigned. Stated another way, the duration of an activity is directly related to how much work is necessary to complete the activity divided by the number of people available to perform the work. However, this does not necessarily mean that doubling available resources will halve the activity’s duration.

If the amount of work is known, along with an estimate of the number of people available to perform that work, then its duration can be estimated, along with efficiency levels, risk, and other external factors. That is, when the amount of work is fixed, the number of labor resources directly affects its duration. For example, if an activity is estimated to require 16 hours of work (2 person-days) and only one full-time equivalent (FTE) employee is available to perform the activity within an 8-hour day, then the duration of the activity will be 2 days. If two FTEs are available to perform the activity, then the duration will be 1 day; if one FTE is available for only 50 percent of the time, the duration will be 4 days.

Productivity or efficiency factors can be applied to specific resources to account for standard output rates, personal experience, or historical productivity. For example, specific persons in a resource group may have more experience performing an activity than other persons in the same group. Productivity varies by the type of work—for example, trees planted per day or drawings produced per week. Workflow also affects productivity: a steady flow of work tends to increase efficiency, whereas a discontinuous flow can introduce inefficiencies. Finally, complex activities may actually require additional duration as more people are assigned to account for greater communication and coordination requirements.

The duration of other types of activities known as fixed-duration activities is not affected by the number of people assigned to perform the work. For example, the number of days required for testing a satellite in a vacuum chamber will be the same regardless of how many engineers are assigned to monitor the testing. Likewise, the duration of a management offsite meeting does not depend on the number of people who attend. In the case of fixed-duration activities, labor resource information is nonetheless vital because the number of people directly affects the work required for the activity and, therefore, the cost. For example, five engineers assigned to a 2-day fixed-duration software coding activity will incur 10 person-days of work at whatever labor rate each engineer earns.

Nonlabor Resources

Significant material and equipment resources should also be specified within a schedule. Material resources are consumables and other supplies that are used to complete a project. Equipment resources may be items such as machines that are installed during the project and become part of the completed project at turnover. Other equipment resources may facilitate a project’s execution but are neither consumed nor turned over in the final product delivery—for example, a rented crane.

Like labor resources, nonlabor resources can be fixed or variable. Fixed nonlabor resources do not vary with an activity’s duration; that is, the same amount of resource is consumed regardless of the activity’s duration. For example, 100 square feet of wood flooring might be needed for a floor, regardless of whether the floor construction takes 2 or 3 days to complete. Variable nonlabor resources may include equipment used during the project such as cranes or testing machines that are not consumed but provide services that vary with the duration of an activity. For example, the longer a testing activity runs, the longer the test equipment is in use.