false hypotheses

Posted in programmer productivity at 10:41 pm by ducky

A theme in Andrew Ko’s papers (which I’ve read a lot of recently) is that a lot of the problems that programmers have is due to making invalid hypotheses. For example, in Designing the Whyline, (referring to an earlier paper, which I didn’t read closely), they say “50% of all errors were due to programmers’ false assumptions in the hypotheses they formed while debugging existing errors.”

It seemed that chasing down an incorrect hypothesis could also chew up a lot of time. I suspect that when coders get stuck, it’s because they have spent too long chasing down the wrong trail.

Yesterday, I had two friends take a quick, casual look at some code to see if they could quickly find where certain output was written out. Both worked forward in the code, starting at main() and tracing through calls to see where control went. At one point in the source, there were three different paths that they could have chosen to trace. The first person chose to follow statement A; the second person chose to follow statement B. Both were reasonable choices, but A happened to be the correct one.

The first person took ten minutes, while the second person spent 30 minutes running off into the weeds chasing statement B. (He eventually gave up, backtracked, and took about ten minutes following statement A to the proper location.)

Implications for programmer productivity measures

The second person took four times as long as the first to complete the task. Was the first person a four times “better” programmer? I don’t think so. From my looking at the code, the second person made a completely legitimate choice. I’m quite happy to believe that on some other task, the first person might make the wrong choice at first and the second person make the right choice.

This makes me even more suspicious of people claiming that there is a huge productivity difference among programmers. The controlled studies that I have seen have all had a very small number of tasks, far too few to make significant generalizations about someone’s long-term abilities.

Furthermore, I think there is sample bias. For a user study, you have to have very simple problems so that people have a chance to finish the allotted tasks in a few hours or less. That favors people who do “breadth-first” analyses of code; who spend a tiny bit of time on one hypothesis, and if that doesn’t quickly give results, move on to the next one.

However, sometimes problems really are gnarly and hairy, and you really do have to stick to one hypothesis for a long time. People who are good at sticking to the one hypothesis through to resolution (without getting discouraged) have value that wouldn’t necessarily be recognized in an academic user study of programmer speed.

How can we reduce false hypotheses?

After a binge of reading Andrew Ko papers last week, I decided to start forcing myself to write down three hypotheses every time I had to make a guess as to why something happened.

In my next substantive coding session, there were four bugs that I worked on. For two of them, I thought of two hypotheses quickly, but then was stumped for a moment as to what I could put for a third… so I put something highly unlikely. In once case, for example, I hypothesized a bug in code that I hadn’t touched in weeks.

Guess what? In both of those cases, it was the “far-fetched” hypothesis that turned out to be true! For example, there was a bug in the code that I hadn’t touched in weeks: I had not updated it to match some code that I’d recently refactored.

While it’s too early for me to say what the long-term effect of writing down three hypotheses will be, in the limited coding I’ve done since I started, it sure feels like I’m doing a much better job of debugging.


  1. Best Webfoot Forward » Breadth-first search said,

    May 28, 2007 at 4:39 pm

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  2. Best Webfoot Forward » Support for "three hypotheses!" said,

    February 11, 2008 at 11:46 pm

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  3. Best Webfoot Forward » My MS thesis is done! said,

    July 24, 2008 at 5:46 pm

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