The statement of scope helps the planner to established estimates using one or more method which fail into two wide categories: empirical modeling and decomposition.
Decomposition method offered a delineation of major software functions which is followed through estimates of either the size or the number of person months needed to implement each function.
Empirical method use empirically derived expressions for time and effort to predict these project quantities. The automated tools can be used to implement a specific empirical model.
Accurate the project estimates mainly make use of at least 2 of the 3 techniques noted as earlier. By comparing and reconciling estimates derived using various methods the planner is more likely to derive an accurate estimate. The Software project estimation is never be an exact science but a combination of systematic techniques and good historical data can improve estimation accuracy