COCOMO
The best known and most transparent cost model COCOMO (COnstructive COst MOdel) was developed by [BOEHM], derived from the analysis of 63 software projects. Boehm proposed three levels of the model; basic, intermediate, detailed. My introduction to COCOMO will focus mainly upon the intermediate model, for coverage of the all levels I strongly suggest reading Boehm’s book.
The COCOMO model is based on the relationships between:
equation 1 development effort is related to system size. MM = a.KDSI b
equation 2 effort and development time. TDEV = c.MM d
MM is the effort in man-moths
KDSI is the number of thousand delivered source instructions.
TDEV is the development time.
The coefficients a, b, c and d are dependant upon the ‘mode’ of development’ which Boehm classified in 3 distinct modes:
- organic - projects involve small teams working in familiar and stable environments. e.g. payroll systems.
- semi-detached - mixture of experience within project teams. In between organic and embedded modes. e.g. interactive banking system.
- embedded - projects that are developed under tight constraints, innovative, complex and have a high volatility of requirements. E.g. nuclear reactor control systems.
Table 2.0
Development Mode a
b
c
d
Organic Semi-detached
Embedded
3.2
3.0
2.8
1.05
1.12
1.20
2.5
2.5
2.5
0.38
0.35
0.32
In the intermediate mode it is possible to adjust the nominal effort obtained from the model by the influence of 15 cost drivers. These drivers allow you to deviate from the nominal figure, where particular projects differ from the ‘average’ project. For example, if the reliability of the software is very high, a factor rating of 1.40 can be assigned to that driver. Once all the factors for each driver have been chosen they are multiplied to arrive at an effort adjustment factor (EAF)
The actual steps in producing an estimate using the intermediate COCOMO model are:
1 Identify the ‘mode’ of development for the new project.
2 Estimate the size of the project in KDSI to derive a nominal effort prediction.
3 Adjust the 15 cost drivers to reflect your project, producing an error adjustment factor.
4 Calculate the predicted project effort using equation 1 and the effort adjustment factor.
5 Calculate the project duration using equation 2.
Assessment of COCOMO
