How to Make Things Faster

Chapter 82. Percentile Specifications

Imagine there’s a task that you execute on your computer hundreds of times every day. And let’s imagine that your tolerance for this task’s duration is 5 seconds per execution, so you formally agree with your application service provider that you’ll be officially satisfied as long as the average response time for this task is 5 seconds or less.

But the first day you use the application, you don’t feel very good about it. You formally object. And so the vendor looks up the numbers for a ten-run period you agreed is representative of why you feel bad about performance. It’s no wonder you’re upset. You circle all the cells where your expectations weren’t met, and you find out that you were disappointed with half the experiences—5 out of 10.

“But,” your service provider insists, “you must agree that we’re meeting your official average response time requirement of 5 seconds or less.” Yes, in fact the average is 5.0 seconds exactly. But you’re not happy, so apparently what you thought you wanted was not what you really wanted.

So, your vendor—benevolently—offers a new type of agreement. “You seem concerned about how often you’re disappointed, so let’s come to an agreement on how often you’re willing to be disappointed.” You agree with the format, and you choose the number 10% as your disappointment threshold. As long as no more than 10% of your ...