When you modify a task that is fixed, you must take into account the effect on other task fields.

For example, let’s say that we have a widget machine that can produce 100 widgets per minute. It always works at the same pace. If we change the number of units, the machine will have to work longer. If we change the duration, the machine will produce more units. And, if we change the amount of work we give the machine, the duration of the task will change. This would be a Fixed Work task.

The default task type is Fixed Units. Let’s say we’re going to send our annual report to the printer to print 500 copies. If we want more copies, the duration will change. If the duration changes, more work will be required. (For example, if the printer only has 4 days instead of 7 to print the report, he will have to work longer hours.) Similarly, if the hours the printer works changes, the duration will change.

Fixed Duration tasks are a bit different. Let’s say that widgets always require 5 hours to set. If more units are added, more work by the machine will be required. If the time a widget takes to set changes, more time in the drying machine will also be required. And, if the work that the drying machine can handle changes, the units produced will be different. Another example is summary tasks: they are always fixed duration because their length depends on how long the subtasks take.