You can increase the accuracy of your estimates for task duration (duration: The total span of active working time that is required to complete a task. This is generally the amount of working time from the start to finish of a task, as defined by the project and resource calendar.) in the following ways.
Use your own past experience and the past experience of others who have done something similar in previous projects. Ask other questions, such as:
Note any differences between the new task and similar tasks done in the past and take into account those differences when estimating a task's duration.
Include the following considerations as you estimate task durations:
You can also derive your estimates by comparing optimistic (optimistic duration: The best-case possibility for the total span of active working time expected for a task, that is, the amount of time from the optimistic start to optimistic finish of a task.), pessimistic (pessimistic duration: The worst-case possibility for the total span of active working time expected for a task, that is, the amount of time from the pessimistic start to pessimistic finish of a task.), and expected durations (expected duration: The total span of active working time expected for a task, that is, the amount of time from the expected start to the expected finish of a task.). This is sometimes called a PERT analysis (PERT analysis: PERT [Program, Evaluation, and Review Technique] analysis is a process by which you evaluate a probable outcome based on three scenarios: best-case, expected-case, and worst-case.).