Distracted by the hard part

Last night I was helping my daughter with calculus homework. I told her that a common mistake was to forget what the original problem was after getting absorbed in sub-problems that have to be solved. I saw this over and over when I taught college.

Then a few minutes later, we both did exactly what I warned her against. She took the answer to a difficult sub-problem to be the final answer. I checked her work and confirmed that it was correct, until I saw we hadn’t actually answered the original question.

As I was waking up this morning, I realized I was about to make the same mistake on a client’s project. The goal was to write software to implement a function f which is a trivial composition of two other functions g and h. These two functions took a lot of work, including a couple levels of code generation. I felt I was done after testing g and h, but I forgot to write tests for f, the very thing I was asked to deliver.

This is a common pattern that goes beyond calculus homework and software development. It’s why checklists are so valuable. We resist checklists because they insult our intelligence, and yet they greatly reduce errors. Experienced people in every field can skip a step, most likely a simple step, without some structure to help them keep track.

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