There are two main approaches used to estimate project work: duration-based and volume-based.
Often schedules are developed with a duration-based approach, mostly due to planning tools limitations.
An alternative, much more powerful approach is based on the volume of work and productivity of assigned resources. Volume-based planning is an intuitive and easy method.
Activity volume is used as initial activity information instead of duration.
Activity volume can be measured in meters, tons, story points, planned work hours, percentages, pages or any other units.
Resource productivity is defined in volume units per hour.
Activity durations are calculated as the volume of work divided by assigned resource productivities.
A worker needs to dig a 100m trench. If his productivity is 5m/h, the duration of this activity would be 20 hours.
A bulldozer is needed to push 200 m3 of sand. We have two resources that could be used for this work: a powerful bulldozer 1 with a productivity of 20 m3/h and an older model bulldozer 2 with a productivity of 10 m3/h.
The duration of the activity will be 10h or 20h, depending on which bulldozer is assigned to this activity.
The assignment is likely to depend on:
- other works need to be completed by bulldozers;
- whether this activity is on a resource critical path or not. If not, we may prefer a less productive/cheaper bulldozer.
Volume-based planning has many advantages:
Projects are often planned (especially in construction and manufacturing) based on the federal, local, industrial or corporate norms and standards. These standards usually refer to resource productivity on the certain activity types per unit of activity volume (costs and materials as well). These norms could be used as a base for planning.
Unlike activity duration activity volume (scope) is a more stable and reliable activity characteristic as it does not depend on assigned resources.
Usually, it is much easier to measure the volume of work to understand activity progress.
When resource supply is changed, activity durations could be automatically adjusted.
It is much easier to complete a What-if analysis and find an optimal resource demand.
If the volume of work is uncertain, the volume range could be used as a base for quantitative risk analysis. The same for productivity. Uncertainties in the productivity of resources also could be taken into account.
Productivity-based scheduling is realised in Spider Project and usually is the preferred way to manage project work.
See how it works in Spider Project: