The basic COCOMO model assumes that the effort and development time are functions of the product size along. however, it is intuitively obvious that a host of other project parameters besides product size affect the effort and development time. Therefore, in order to obtain an accurate estimation of the effort and project duration, the effect of these parameters must be taken into account.

The intermediate COCOMO model recognizes this fact and refines the initial estimate obtained by the basic COCOMO by using a set of 15 cost drivers (Multipliers). For example, if modern programming practices are used, the initial estimates are scaled downwards by multiplying with a cost driver having a value less than 1. If there are stringent reliability on the software product, this initial estimate is scaled upwards. Boehm requires the project manager to rate these different parameters (i.e. attributes depeding on these ratings, he suggested that the appropriate cost driver values should depending on these ratings, he suggested that the appropriate cost driver values should be multiplied by the initial obtained by using the basic COCOMO. In general, the cost drivers can be classified as being attributes of the following items:

The cost drivers are grouped into four categories:

1.    Product attributes
(a)    Required software reliability (RELY)
(b)    Database size (DATA)
(c)    Product complexity (CPLX)

2.    Computer attributes
(a)    Execution time constraint (TIME)
(b)    Main storage constraint (STOR)
(c)    Virtual machine volatility (VIRT)
(d)    Computer turnaround time (TURN)

3.    Personnel attributes
(a)    Analyst capability (ACAP)
(b)    Application experience (AEXP)
(c)    Programmer capability (PCAP)
(d)    Virtual machine experience (VEXP
(e)    Programming language experience (LEXP)

4.    Project attributes
(a)    Modern programming practices (MODP)
(b)    Use of software tools (TOOL)
(c)    Required development schedule (SCED)

Each cost driver is rated for a given project environment. The rating uses a scale “very low, nominal, high very high, extra high which describes to what extent the cost driver lies to the project being estimated. gives the multiplier values for the 15 cost drivers and their rating as provided by Boehm.

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