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On January 24, 2014, In Estimating Duration and Cost by Admin

The COnstructive COst MOdel (COCOMO) is perhaps the most extensively used estimating technique. It is a regression-based model developed by Dr. Barry W. Boehm when he was at TRW in the early 1970s. 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. Regression analysis was then used to develop exponential equations that best explain the relationship between the scattered data points.

● The use of regression to make quantitative predictions of one variable from the values of another.

● A statistical technique used to find relationships between variables for the purpose of predicting future values.

● Procedures for finding the mathematical function that best describes the relationship between a dependent variable and one or more independent variables. In linear regression, the relationship is constrained to be a straight line, and least-squares analysis is used to determine the best fit.

In many regression, the dependent variable is considered to depend on more than a single independent variable.

Figure (a) shows a scattergram on the left and an attempt at fitting a line through the data points on the right. In this example, cavities are expected to increase with grams of sugar consumed.

COCOMO refers to three modes, which categorize the complication of the system and the development environmqent (see Table 1).

The organic mode is typified by systems 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.

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

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 complex interfaces, including those with hardware and with customers.

This level uses only size and mode to determine the effort and schedule. It is useful for fast, rough estimates of small to medium-size projects.

This level uses size, mode, and 15 additional variables to determine effort. The additional variables are called "cost drivers" and relate to product, personnel, computer and project attributes that will result in more effort or less effort required for the software project. The product of the cost drivers is known as the environmental adjustment factor (EAF).

This level builds upon intermediate COCOMO by introducing the additional capabilities of phase-sensitive effort multipliers and a three-level product hierarchy. The intermediate level may be tailored to phase and product level to achieve the detailed level. An example of phase-sensitive effort multipliers would be consideration of memory constraints when attempting to estimate the coding or testing phases of a project. At the same time, though, memory size may not affect the effort or cost of the analysis phase. This will become more obvious after the effort multipliers (or cost drivers) are described. Phase-sensitive multipliers are usually reserved for use in mature organizations and need the use of an automated tool.

A three-level product hierarchy consists of system, subsystem, and module, much like the arrangement of a WBS. Large projects may be decomposed into at least three levels so that each of the cost drivers introduced in intermediate COCOMO is assigned to that level likely to be most influenced by variations in the cost driver. For instance, an engineer's language experience may apply only at the module level, an analyst's capability may apply at a subsystem level and a module level, and required reliability may apply at the system, subsystem, and module level. As with the phase-sensitive multipliers, this will make more sense after the cost drivers are described. As with the phase-sensitive multipliers, mature organizations with automated tools are the heaviest users.

KLOC is the only input variable. An exponential formula is used to calculate effort, as shown in Box 1.

As pointed out by Dr. Frailey, effort is measured in staff-months (19 days per month or 152 working hours per month), the constants a and b can be determined by a curve fit procedure (regression analysis), matching project data to the equation. Most organizations do not have sufficient data to perform such an analysis and begin by using Boehm's three levels of difficulty that seem to characterize many software projects. Box 2 shows the basic formulas; Table 2 lists the effort and development time formulas for each mode.

The same-size project yields different amounts of effort when it is considered to be of different modes:

Suppose that a project was estimated to be 200 KLOC. Putting that data into the formula

Effort = a x (Size)

Effort for the organic mode would be estimated at 2.4 x (200)

Effort for the semidetached mode would be estimated at 3.0 x (200)

After effort is estimated, an exponential formula is also used to calculate a project duration, or completion time (time to develop, TDEV). Project duration is expressed in months.

Boehm devised three formulas (see Box 3) to be used for development time in the same fashion as he did with effort.

When effort (E) and development time (TDEV) are known, the average staff size (SS) to complete the project may be calculated, as shown in Box 4.

When average staff size (SS) is known, the productivity level may be calculated, as shown in Box 5.

Two examples of basic COCOMO follow. One is simple, and the other is of medium complexity.

A development project is sized at 7.5 KLOC and is evaluated as being simplein the organic mode.

The basic COCOMO equation for effort (E) in staff-months (SM) is:

Effort (SM) = 2.4(KLOC)

Development time (TDEV) can also be found by using the basic COCOMO formulas:

TDEV = 2.5(SM)

The average number of staff members (S):

Staff = Effort TDEV = 20 staff-months 8 months = 2.5 staff members on average

The productivity rate (P):

Productivity = Size Effort = 7,500 LOC 20 staff-months = 375 LOC/staff-month

A development project is estimated to be about 55 KLOC when complete and is believed to be of medium complexity. It will be a Web-enabled system with a robust back-end database. It is assumed to be in the semidetached mode.

For a rough estimate of the effort that will be required to complete the project, use the formula:

E (effort in staff-months) = 3.0(KLOC)

E (effort in staff-months) = 3.0(55)

E = 3.0(88.96)

E = 267 staff-months

To determine how long it will take to complete the project, use the formula:

TDEV = 2.5 x (E)

TDEV = 2.5 x (267)

TDEV = 2.5(7.07)

TDEV = 17.67 months

To obtain a rough estimate of how many developers will be needed, use the formula:

S (average staff) = effort

`÷`

TDEVS (average staff) = 267

`÷`

17.67S (average staff) = 15.11

To determine a rough estimate of the productivity rate, use the formula:

P (productivity) = size

`÷`

effortP (productivity) = 55,000

`÷`

267P (productivity) = 206 LOC/staff-month

Basic COCOMO offers a way to calculate a set of quick estimates for effort, development time, staffing, and productivity rates, given knowledge only of size and mode. It can be arrived at with no more than a calculator. However, you get what you pay for. It doesn't take much to derive effort using the basic level, and the results won't be worth much more than a very, very rough estimate. To refine the estimation process, Boehm gives guidance in "tuning" via what he calls a complexity adjustment factor described in intermediate COCOMO.