Software Cost Estimation in Software Engineering

Software cost estimation : Predicting the resources required for a software development process

Fundamental estimation questions

How much effort is required to complete an activity?
How much calendar time is needed to complete an activity?
What is the total cost of an activity?
Project estimation and scheduling and interleaved management activities

Software cost components

Hardware and software costs
Travel and training costs
Effort costs (the dominant factor in most projects)
- Salaries of engineers involved in the project
- Social and insurance costs
Effort costs must take overheads into account
Costs of building, heating, lighting
Costs of networking and communications
Costs of shared facilities (e.g library, staff restaurant, etc.)

Costing and pricing

Estimates are made to discover the cost, to the developer, of producing a software system
There is not a simple relationship between the development cost and the price charged to the customer
Broader organisational, economic, political and business considerations influence the price charged

Programmer productivity

A measure of the rate at which individual engineers involved in software development produce software and associated documentation
Not quality-oriented although quality assurance is a factor in productivity assessment
Essentially, we want to measure useful functionality produced per time unit Productivity measures
Size related measures based on some output from the software process. This may be lines of delivered source code, object code instructions, etc.
Function-related measures based on an estimate of the functionality of the delivered software. Function points are the best known of this type of measure

Measurement problems

Estimating the size of the measure
Estimating the total number of programmer months which have elapsed
Estimating contractor productivity (e.g. documentation team) and incorporating this estimate in overall estimate

What’s a line of code?

The measure was first proposed when programs were typed on cards with one line per card

How does this correspond to statements as in Java which can span several lines or where there can be several statements on one line

What programs should be counted as part of the system?

Assumes linear relationship between system size and volume of documentation

Productivity comparisons

The lower level the language, the more productive the programmer

The same functionality takes more code to implement in a lower level language than in a high-level language

The more verbose the programmer, the higher the productivity

Measures of productivity based on lines of code suggest that programmers who write verbose code are more productive than programmers who write compact code

Function points

Based on a combination of program characteristics
External Inputs and Outputs
User Interactions
External Interfaces
Files Used by the System

A weight is associated with each of these

The function point count is computed by multiplying each raw count by the weight and summing all values

Function point count modified by complexity of the project

FPs can be used to estimate LOC depending on the average number of LOC per FP for a given language

LOC = AVC * number of function points

AVC is a language-dependent factor varying from 200-300 for assemble language to 2-40 for a 4GL

Object points

Object points are an alternative function related measure to function points when 4Gls or similar languages are used for development
Object points are NOT the same as object classes
The number of object points in a program is a weighted estimate of
- The number of separate screens that are displayed
- The number of reports that are produced by the system
- The number of 3GL modules that must be developed to supplement the 4GL code

Object point estimation

Object points are easier to estimate from a specification than function points as they are simply concerned with screens, reports and 3GL modules

They can therefore be estimated at an early point in the development process. At this stage, it is very difficult to estimate the number of lines of code in a system

Productivity estimates

Real time embedded systems, 40-160 LOC/P-month
Systems programs , 150-400 LOC/P-month
Commercial applications, 200-800 LOC/P-month
In object points, productivity has been measured between 4 and 50 object points/month depending on tool support and developer capability

Software Cost Estimation – Quality and productivity

All metrics based on volume/unit time are flawed because they do not take quality into account
Productivity may generally be increased at the cost of quality
It is not clear how productivity/quality metrics are related
If change is constant then an approach based on counting lines of code is not meaningful

Estimation techniques

There is no simple way to make an accurate estimate of the effort required to develop a software system
Initial estimates are based on inadequate information in a user requirements definition
The software may run on unfamiliar computers or use new technology
The people in the project may be unknown
Project cost estimates may be self-fulfilling
The estimate defines the budget and the product is adjusted to meet the budget