On the Compatibility between Full Function Points and Ifpug Function Points

Release 1.0 of the Full Function Points measurement method was proposed in 1997 to measure the functional size of real-time or embedded software. Since then, field tests have shown the applicability and usefulness of this measurement method not only for real-time or embedded software, but also for other types of software like system software and MIS software. This paper investigates the issue of measurement compatibility between the designs of both the FFP and IFPUG measurement methods. Such compatibility is required to perform mathematical operations involving results from both methods and mixing them into a single functional size measure. The compatibility of both measurement objects and measurement processes is analyzed and the accuracy of their aggregation function is identified as being dependent on the level of granularity at which the measurement functions are applied. Comparing the two approaches, we find that the precision of this aggregation function corresponds to the lowest common denominator of the two approaches. 1. Context Release 1.0 of Full Function Points (FFP) was proposed in 1997 as a functional size measurement method derived from the International Function Point Users Group (IFPUG) method. Strictly speaking, the Full Function Points measurement method is a conceptual superset of the IFPUG method, as documented in [1, p. 15]. The extensions proposed by the FFP measurement method were originally intended to extend the applicability of the IFPUG method to the field of realtime and embedded software, as presented in [3, 4]. The FFP approach suggests adding the points obtained by the IFPUG method to the points obtained through the FFP extensions for the measurement of control data and control transactions. The following issue can then be raised: Is it conceptually sound to add the points obtained through the FFP extension to the points obtained through the IFPUG approach? This paper explores this issue and shows that the two approaches offer a degree of compatibility which allows FFP and IFPUG functional size figures to be combined into a single size figure. The comparison between the two approaches is based on version 4.0 of the IFPUG method [2], and the extensions proposed in version 1.0 of the FFP method [1]. These are the two most recent versions currently available. 2. A common framework for comparison A functional size measurement method consists of applying a set of rules and procedures to a given software; the result of the application of these rules and procedures is a numerical figure representing the functional size of the software. In both the FFP and IFPUG Function Points approaches, the rules and procedures are described in a document referred to as the “Counting Practices Manual”. The essence of each approach is described in these measurement standards documents. A closer look at the contents of these manuals shows that both approaches: • On the one hand, are intended to be independent of the implementation decisions embedded in the operational artifacts of the software to be measured, and, • On the other hand, involve an obligation to apply measurement rules and procedures to some visible artifacts related to the software to be measured. A more detailed view of the measurement approach proposed by FFP and by IFPUG Function Points is presented in Figure 1 below. This figure illustrates the common framework for comparing FFP and IFPUG Function Points. Figure 1 – A common framework for comparing FFP and IFPUG Function Points This framework illustrates that, prior to applying the measurement rules and procedures, the software to be measured must be mapped onto a specific software model that captures the concepts and definitions required for a functional size measurement exercise. Although this model is not explicitly defined as such in the Counting Practices Manual of each approach, both rely on an implicit model of the software to be measured. It is on the objects of this implicit software model that a) the software to be measured is mapped, and b) the rules and procedures of each approach are applied in order to produce a numerical figure representing the functional size of the software. Therefore, two distinct and related processes are necessary to measure the functional size of software: mapping of the artifacts of the software to be measured onto an implicit software model and then measuring specific features of this software model. The mapping process takes as input the artifacts of the software to be measured (as they are found/documented within the organization) and produces as output an instance of a software model. This instantiated software model is defined by the concepts and definitions found in each approach. Next, the measurement process takes as input the instantiated software model and produces as output the numerical figure representing the functional size of the software model. By convention, this numerical figure is then extended to represent the functional size of the software itself. The functional size figure is therefore derived by applying the documented set of rules and procedures described in each approach. The framework presented in Figure 1 will be used as a common basis for comparing FFP and IFPUG Function Points. For the two approaches to be compatible, their software models must be compatible and their measurement processes must be compatible. Measurement Process Rules and Procedures Instance of a software model Concepts and Definitions Functional size of the software model Mapping Process Software to be measured &RXQWLQJ 3UDFWLFHV 0DQXDO 3. Comparing the software models In order to compare the software models generated by the FFP and the IFPUG Function Point approaches, it is necessary to extract from each approach the concepts that characterize them. Five concepts characterize the software model of the FFP approach, while four concepts characterize the software model of the IFPUG approach. These are presented in Table 1 below. The software model of each approach will be deemed to be compatible if all concepts are compatible. Table 1 – Concepts characterizing the software models measured by FFP and IFPUG Function Points