If certain tasks have been carried out before, it is common to assume that the new tasks will have similar durations.
This is OK provided the individual has concluded there are no special circumstances.
Such a practice is often a good starting point and useful for the quick submission of estimates.
Old schedules or generic schedules templates can be used as a starting point or a simple checklist to produce an updated version for the new project.
Don’t forget that a lot of historical data is in peoples’ heads!
If the experience exists use it as much as possible.
It is wise to know what your personnel (both new and old) have worked on in the past.
With many records now being computerised it makes it easier to search data bases for possible useful information.
Data bases may not only be limited to your work place. There is the internet, libraries etc.
It is often the case that some tasks are recognised as taking a particular amount of time.
This can be particularly useful when there is a direct relationship between tasks.
For example, it may be that one task is dependant upon another and they always have a fixed relationship.
Task ‘B’ may always take twice as long as task ‘A’ for instance.
So, if you know the duration for task ‘A’ is 2 weeks it will mean the duration for task ‘B’ will be 4 weeks etc.
Some tasks may be well described by a more complex formulae based upon building up years of historical data and trends.
This may be a common practice in some sectors.
Basically, consider carefully where you may be able to get data that could help.