In this step we'll look at something named the "Value Adjustment
Factor", or VAF. But first, a warning about the VAF.
A warning about the VAF
Before we tell you what the VAF is, let us first give you a
warning about it: To the best of our knowledge, most users don't use
it.
We can't tell you much about the history of the VAF, but what we
can
tell from the conversations we've had with many other users is
that
they don't use the VAF. This stems from at least two reasons
that we
can
determine:
- Most users count function points to derive a number that they
can plug into another piece of software to determine a cost
estimate.
Those other software applications usually have their own
equivalent
of
the VAF, and in fact, instruct you to supply the "raw
FP count". So,
in
this case, the VAF competes against these vendor
tools.
- Some users don't feel that the GSCs are flexible enough. I
tend
to agree, and I think it's an easy argument. When you look at the
math below, you'll see that for two applications under
consideration,
if both start with the same function point count -
let's say 1,000
FPs
- after adjustments the hardest application in
the world would be
rated
at 1,350 FPs, and the easiest possible
application would be rated at
650 FPs when adjusted. Let's say the
hardest application in the
world
had to run on 10 different operating
systems in 15 languages and be
distributed electronically to 1
million users, and the easiest
would
be
written in Microsoft Access,
run on Windows, and be used by only one
user, the author of the
program. Do you really think the first
application is only about
twice as hard to deliver as the second?
No, I
certainly don't, and
this is why I don't use the VAF.
All that being said, we'll give you the quick VAF introduction so
you can make a decision on your own. If nothing else, we find it
nice
that someone has taken the time to come up with these 14
categories.
It
helps our thinking in other areas.
VAF Introduction
Here are a few facts and definitions to get the ball rolling:
- The Value Adjustment Factor (VAF) consists of 14 "General
System
Characteristics", or GSCs.
- These GSCs represent characteristics of the application under
consideration. Each is weighted on a scale from 0 (low) to 5
(high).
- When you sum up the values of these 14 GSCs you get a value
named
"Total Degree of Influence", or TDI. As you can see from the math
the TDI can vary from 0 (when all GSCs are low) to 35 (when all GSCs
are high).
Before getting into the VAF formula, let us quickly list the 14
GSCs:
- Data Communication
- Distributed data processing
- Performance
- Heavily used configuration
- Transaction rate
- Online data entry
- End user efficiency
- Online update
- Complex processing
- Reusability
- Installation ease
- Operational ease
- Multiple sites
- Facilitate change
Given this background information, you can see with the following
formula:
VAF = (TDI*0.01) + 0.65
that the VAF can vary in range from 0.65 (when all GSCs are low)
to
1.35 (when all GSCs are high). In the next section you'll see that
the VAF is applied directly to the FP count to determine the
"Adjusted
Function Point Count".