Thursday, July 10, 2008

Zero Defect Software Development (ZDSD)

by Steve Pavlina,
Dexterity Software

The basic tenet of ZDSD is this:

Maintain your product in what you believe to be a defect-free state throughout the development process. This sounds simple, but it
is a rare practice. The most common approach is to delay major testing until the final QA phase of software development, where
defects are often discovered for the first time. Most bugs are not detected or fixed until long after their introduction. The
longer a defect remains, the harder it is to fix. On large software products, each stage of development that a defect survives
will increase the cost of fixing the defect by ten to fifty times. A defect introduced in the design phase can cost hundreds of
times more to fix in the testing phase than it would if fixed immediately after its introduction.

By focusing on product quality throughout the development lifecycle, you will actually complete products faster than if you didn't
pay attention to quality until the end of the project. The general rule of software quality is counter-intuitive: Improving
quality actually reduces development time. This is because you eliminate all the time spent fixing bugs and reworking code, which
can account for as much as 50% of development costs on a large project. The typical programmer writes between eight and twenty
lines of code a day; the rest of the day is usually spent on debugging. ZDSD shortens schedules by eliminating most debugging
time. Extensive studies done at NASA, IBM, and elsewhere have shown that better QA leads to shorter schedules.

An IBM study concluded that software projects that make quality a top priority typically have the shortest schedules, the highest
productivity, and even the best sales.

Here are the basic rules of ZDSD:

1. Test your product every day as you develop it, and fix defects as soon as you find them.Apply the daily build and smoke test.
At the end of every day you work on your project, build the current version of your software, and test it for basic functionality.
Microsoft enforces this policy religiously, using large teams to build each project on a daily basis. A programmer whose code
breaks the build may be called in the middle of the night and must go back to work to fix the problem immediately. For shareware
authors working on small projects, this is far easier. At the end of each day, test your program for at least ten minutes. Make a
list of anything you would consider a "defect," and resolve to fix all defects before implementing any new features. Once you find
a defect, fixing it becomes your number one priority, and you avoid writing any new code until the defect is 100% eliminated.

2. Review your code regularly. When most people think of QA, they think of testing, but testing is actually one of the least cost-
effective strategies for finding bugs. The most rigorous testing will typically find less than 60% of all bugs in a program, and
there are certain types of bugs that testing will rarely find. Studies conducted at many large software organizations have
concluded that code inspections are far more cost-effective than testing. A NASA study found that code reading detected almost
twice as many defects per hour as testing. Whenever you've added a few hundred lines of new code to your project, set aside an
hour or two to read over your work and look for mistakes. One hour of code review is equivalent to two or more hours of methodical
testing. As you gain experience, keep a list of the types of defects you find, and run down your list whenever reviewing new code.
To find even more defects, have someone else read your code as well.

3. Assume full responsibility for every bug. 95% of all software defects are caused by the programmer. Only 1% of defects are
hardware errors, and the remaining 4% are caused by the compiler, the OS, or other software. Never dismiss a potential bug; find
out the exact cause of any anomaly. When the Mars probe suffered serious software glitches during its mission, it was learned that
the same glitch had occurred only once during testing on earth, but the engineers dismissed it as a temporary hardware hiccup.
Unless your hardware drinks soda, it does not hiccup.

4. Handle change effectively. You will always think of great new features to add after you have started coding. Carefully consider
how each change will impact your pre-existing code. Poor integration of unanticipated features is a major cause of defects.

5. Set QA objectives at the beginning of every project. Studies have shown that developers who set reasonable QA goals will
usually achieve them. Decide in advance if your product must be fast, small, feature-rich, intuitive, scalable, etc. Then
prioritize those objectives. When designing the interface code for an upcoming game, I decided that my top three priorities were
to make it beginner-intuitive, fast, and fun, in that order. Consequently, my game's interface isn't as graphically rich as other
games, but it is easier to use and faster than any other game of its type. Whenever you have to make a design decision, keep your
objectives in mind. If you do not set clear QA goals, then you are doomed to accept the results of random chance.

6. Don't rush debugging work. Fully 50% of all bug fixes are done incorrectly the first time, often introducing new bugs in the
process. Never experiment by simply changing "x-1" to "x+1" to see if that will do the trick. Take the time to understand the
source of the bug. Years ago when I was a boy scout and had to put out a campfire, the Scoutmaster would sometimes test my
thoroughness by asking me to put my hand in the ashes. I learned very quickly how to put out a fire so well that I had complete
confidence it was 100% extinguished. When you find a defect, it means your code is on fire. As long as the defect remains, any new
code you write will add fuel to that fire. Whenever you find a defect, drop everything to fix it, and don't move on until you are
100% confident that your fix is correct. If you don't take the time to do it right the first time, when will you find the time to
do it over?

7. Learn from every bug; each one represents a mistake that you made. Learn why you made each mistake, and see if you can change
something about your development practices to eliminate it. Over the years I have adopted many simple coding practices that allow
me to avoid common bugs that used to plague me. There are many types of bugs that I now never encounter because my coding style
makes it physically impossible for me to introduce them.

Each of these rules represents a simple concept, but their combined benefits are significant. You will achieve higher progress
visibility, avoiding the situation of being "99% done" for the last 80% of your development time. Higher quality will make your
products easier to maintain and less expensive to support. You will spend less time debugging old code and more time writing new
code. And most importantly, it actually takes less time to write high-quality code than it does to write low-quality code, so you
will save a great deal of time on overall development. If you have never developed products with a zero-defect philosophy from day
one, its adoption can reduce your development time for new products by 30% or more while simultaneously improving product quality



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