One of the thrills of software development is how much power is placed into the hands, literally, of a single engineer. Software takes work, and lots of it. But thanks to frameworks of reusable code, individuals are consistently able to “outbuild” the work of our predecessors, while exerting the same or less effort.

Consider the work that went into the first word processors and web browsers, whose work can now be mimicked by a Mac developer who knows how to embed NSTextView or or WebView. Imagine the time that early game developers invested in rudimentary sprite drawing, animation, and collision, which is now achieved easily in iOS games with open source packages such as cocos2d.

Still, building truly great software continues to elude most developers. It’s easy to assume that a large amount of time invested in a project, combined with an impressive outcome, is the key to a successful product. But it’s not so simple.

The ninety-ninety rule is often cited when describing the challenge of finessing a product after most of the hard work has seemingly been done:

“The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.” — Tom Cargill

I agree with this perspective, and I cite it often when discussing software with others. The rule as stated is in terms of “code” but is also applicable to all the visual design, user experience, marketing, and positioning of a product. I think the rule is also recursive: whatever is missing can always be split up such that an “easy” 90% of the job can be done, leaving an elusive 10% of finesse work.

Many developers mistakenly assume that after they’ve put a ton of work into a project, and it achieves some impressive feat, that their work is done. When you focus on the enormity of work completed thus far, it’s tempting to pat yourself on the back and call it a day.

But customers don’t care about the hard work that went into the 90%. They only notice the flaws or omissions in the 10%. Imagine if Michelangelo’s statue of David was perfect in every detail, except that where David’s face should be, a clump of ragged, unfinished marble lay crudely in its place. Instead of marveling at the exquisite details of David’s feet, torso, and arms, viewers would reject it as unfinished and clumsy.

An important takeaway for software developers is that the missing 10%, or the missing one-tenth of 10%, may be something that will take a great deal of work to get right, but it may be something you simply overlooked the importance of.

You might build an impressive skyscraper projecting 40 stories into the sky. The architecture, interior design, plumbing, and electrical work may all be superior to that of your peers. But if some blindness in your product vision prevents you from adding windows, the product will never sell. It doesn’t matter how much time or effort went into this masterpiece: nobody will live in a windowless skyscraper.

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