Task effort should be estimated in man days, for example.
This is because the actual duration will be dependant upon the actual level of resource attached to the task and other constraints.
For a task using 4 men and with a duration of 5 weeks the total effort = 4 x 5 = 20 man weeks.
Contingency should not be introduced to each task but instead applied globally to cover the budget for the entire project.
If each task is estimated as 10% over the requirement the total project will not only cost 10% more but will be 10% longer.
Contingency should be added for particular risks and any resource can be used globally by the Project Manager.
Tasks should be estimated in an honest manner.
Get the input of interested parties and those doing the task for good estimations.
This will provide more realistic estimates and gain the commitment of those carrying out the tasks.
Task effort and durations must take into account the skill level and experience of the staff.
Naturally, less skilful individuals are likely to take longer for a particular task and may carry it out to a lower standard.
Be wary of the need for training. Particular tasks may require individuals with specific qualifications.
If necessary, any significant factors and assumptions or constraints should be documented.
The schedule should be checked before circulation.
Get the input from the key interested parties.
In addition, it is useful to check with the client (Project Sponsor) as to the degree of accuracy required prior to preparing an estimate.
This should be documented together along with the process adopted by the Project Manager in trying to achieve the required degree of accuracy.
Remember also to agree with the Project Sponsor the time needed to review requirements in order to prepare the estimate.
This can take a considerable time to do it properly depending upon the degree of accuracy required.
In general the Project Manager should consider quantity, quality, cost and duration for all resource required.
Also, think about how the resource may be shared with other projects.