Change  sizing   

This technique is used when a project encompasses the use of existing software which must be modified in some way as part of a project.  The planner estimates the number and kind that are reuse, adding code, changing code, and deleting code of modifications that must be accomplished. Using the effort ratio for each kind of change, the size of the change will be estimated.

Myers and Putnam suggest that the results of each of the sizing approaches noted above be combined statistically to build a 3 points or expected value estimate. This is accomplished through establishing optimistic low most likely and pessimistic high values for size and combining those using equations.