Keeping task duration fixed when assigning resources

I have a task in my project that lasts a certain amount of time, regardless of how many resources are assigned to it. When I add resources to the task, the duration shrinks! How can I change this?

If you don’t want a task’s duration to change when you add resources, you’ll need to tell OmniPlan that you want to keep the duration fixed. After selecting your task, open the Task: Assignments Inspector. Under where it says “When assigning resources,” select the Adjust task effort option. That’s it! Future resource adjustments to this task will now alter the effort for the task, rather than the duration.