The Construction Cost Model (COCOMO) is arguably the most widely used estimating technique. It is a regression-based model developed by Dr. Barry W. Boehm. He began by analyzing 63 software projects of various types. The projects were observed for actual size (LOC), actual effort expended, and actual schedule duration. Boehm postulated that there are essentially three important classes of software products:

1.    Organic
2.    Semidetached
3.    Embedded

These three product classes correspond to application, utility and system programs, respectively. In order to classify a product into any of the three proposed classes, Boehm requires us to take into consideration the characteristics of the product as well as those of the development team. Data processing and scientific programs are considered to be application programs. Compeers, linkers, etc., are utility programs. Operation systems and real-time system programs, etc. are system programs System programs interact directly with the hardware and typically involve timing constraints and concurrent processing.

Brooks [1975] states that utility programs are three times as difficult to write as application programs and system programs are three times as difficult as utility programs. Thus, according to Brooks the relative levels of product complexity for the there categories of products are in the ratio 1 : 3 : 9 for application utility and system programs.

Boehm’s definitions of organic, semidetached, and embedded systems are elaborated below:

Organic. Relatively small groups work in a familiar environment to develop well understood application programs.

The organic mode is typified by stems such as payroll, inventory, and scientific calculation. Other characterizations are that the project team is small, little innovation is required, constraints and deadlines are few, and the development environment is stable.

Semidetached. Project teams consist of a misture of experienced and inexperienced staff. Team memebers have limited experience on related systems and may be unfamiliar with some aspects of the system being developed.

The semidetached mode is typified by utility systems such as compilers, database systems, and editors. Other charcterizations are that the project team is medium-size, some innovation is required, constrainst and deadlines are moderate, and the development environment is somewhat fluid.

Embedded. The software is strongly coupled to compels hardware, or where there are tight regulations governing the operational procedures.

The embedded mode is typified by real-time systems such as those for air traffic control, ATMs, or weapon systems. Other characterizations are that the project team is large, a great deal of innovation is required, constraints and deadlines are tight, and the development environment consists of many compels interfaces, including those with hardware and with customers.

For the three categories of product complexity, Boehm provides equations to predict the project duration (programmer-months) from the size estimation given in kilo lines of code, (KLOC). He derived these equations by examining historical data from a large number of actual projects. According to Boehm, software cost estimation is done through three stages : basic COCOMO, intermediate COCOMO, and complete COCOMO.

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