Projects, generally speaking, consist of tasks carried out in a certain order. With this chain of events, we can create a critical path of tasks and estimate progress toward completion of a project.
A project task should represent work done by one individual, should be completed in a reasonable amount of time and should have a tangible work product as an output.
Work products have value
Project tasks have work products -- deliverables that the project manager can review. It could be a report, a computer subroutine or a new machine part. A value is assigned to the work product of the project task. In most cases, the value will be the cost of producing the work product, but in other cases it might be the value of the work product, which is a more difficult approach.
A work product (e.g., software subroutines) is often part of a larger deliverable, such as a payroll program.
With tasks, 'percent complete' isn't actually effective
You have all been there. Someone says they are 90% done with a project task one week, then 92% done the next … Hence the expression "The first 90% of a task goes much faster than the last 90% of the task." Precision goes out the window. With the alternative method, earned value (EV), either you are done with a project task or you are not. There is zero percent completion until a project task is done.
Why earned value is better
Earned value is a concept I encourage everyone to use. The EV of the project is equal to the value of all the tasks in the project. Suppose the EV for your project is $200,000 and that a given task is valued at $40,000:
If the project task is completed, $40,000 is earned, and if this is the first project task, the project has made 20% of the earned value ($40,000/$200,000). If the project task runs late, none of the EV is counted, because you only get credit when project tasks are done. EV is the core concept in earned value management (EVM), a widely used project management technique.
Note that percent complete is never discussed while a task is in progress, because earned value is only applied to completed tasks. The newly defined percent complete should only refer to the percentage of value earned.