Information Technology Reference
In-Depth Information
Fig. 2. Propagation of residual defects (bugs) as a result of defect insertion and
defect removal rates during several stages of the software production process, ac-
cording to a NASA research on high assurance software for flight control units for
each 1000 lines of code (TLOC) [12].
which cannot be used for product differentiation (core functionality accord-
ing to UNISIG subset 026 [1]). Since all or at least most of the documents
are created by humans, there is always the “human factor” involved, causing
ambiguities and therefore divergent results.
Herbert Klaeren refers to reports in his lecture [9], which have found an
average of 25 errors per 1000 Lines Of programming Code (TLOC) for newly
produced software. The topic “Code Complete” by Steve McConnell has a
brief section about errors to be expected. He basically says that there is a
wide range [10]:
(a)
Industry Average: “about 15 - 50 errors per 1000 lines of delivered code.”
He further says this is usually representative of code that has some level
of structured programming behind it, but probably includes a mix of coding
techniques.
(b)
Microsoft Applications: “about 10 - 20 defects per 1000 lines of code dur-
ing in-house testing, and 0.5 defect per TLOC in released products [10].”
He attributes this to a combination of code-reading techniques and inde-
pendent testing.
(c)
“Harlan Mills pioneered a so called 'clean room development', a technique
that has been able to achieve rates as low as 3 defects per 1000 lines of code
during in-house testing and 0.1 defect per 1000 lines of code in released
product (Cobb and Mills 1990 [11]). A few projects - for example, the
space-shuttle software - have achieved a level of 0 defects in 500,000 lines
of code using a system of formal development methods, peer reviews, and
statistical testing.”
However the U.S. space shuttle software program came at a cost level of
about U.S. $ 1,000 per line of code (3 million LOC
3 billion U.S. $ [9],
cost basis 1978), not typical for the railway sector, which is more in a range
between 30
∼
per LOC for non-safety applications and up to 100
for SIL
e
e
3-4 quality (SIL: Safety Integrity Level) levels.
