Backward Estimation is a general estimation method for defining and calculating time length of tasks through a backward analysis. The method is used in cases when end date of a task is set even before the task is defined. For example, such a task can be a component of a larger activity or project that has a definite completion date. Then the task’s end-date equals the project’s completion date.
The advantage of backward estimation is that it lets consider desired outputs and available resources in advance before a task is started. A backward estimate includes some key figures and calculations that help forecast task duration and set an exact start date. The disadvantage is that the method creates a time estimate dependent on high-level timelines and schedules. The method is not applicable if a task has no definite completion date.
Backward estimation can be presented as a process that consists of the following steps:
- Define a task to be estimated
- Check if this task has a preset end-date
- If not, check whether the task belongs to a larger activity or project
- If so, find out completion date of the project
- Regard the task’s end-date as the project’s completion date
- Create a high-level timeline for the task
- Add the end-date to the timeline
- Go backward to the present time to outline the task’s start date
- Calculate a rough duration estimate based upon the task’s end-date and the outlined start date
- Check if goals of the task can be achieved during the roughly estimated period
- If so, approve the outlined start date
- If not, understand what additional time is required
- Determine the task’s duration.