Function points is basic data from which productivity metrics could be computed.
FP data is used in two ways:
as an estimation variable that is used to "size" each element of the software,
as baseline metrics collected from past projects and used in conjuction with estimation variables to develop cost and effort projections.
The approach is to identify and count a number of unique function types:
external inputs (e.g. file names)
external outputs (e.g. reports, messages)
queries (interactive inputs needing a response)
external files or interfaces (files shared with other software systems)
internal files (invisible outside the system)
Each of these is then individually assessed for complexity and given a weighting value which varies from 3 (for simple external inputs) to 15 (for complex internal files).
Unadjusted function points ( UFP ) is calculated as follows :
The sum of all the occurrences is computed by multiplying each function count with a weighting and then adding up all the values. The weights are based on the complexity of the feature being counted.
In order to find adjusted FP, UFP is multiplied by technical complexity factor ( TCF ) which can be calculated by the formula :