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Comment by mabbo | Hacker News Reader

The 747 ping pong ball estimation problem is a Fermi problem, where the goal is to get the right order of magnitude of the answer. Sure.

But "why are manhole covers round?" is not. It's a stupid "have you ever heard this clever fact" question that's often designed just to figure out if you're clever and/or lucky vs testing actual knowledge and skill.

If your aim in interviewing is to fill the team with people who love being clever then it's a great technique. But in my view, it's better to interview for the skills the job requires.

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I worked and interviewed a lot of people in business consulting. We did ask what you call Fermi problems as they are surprisingly representative of the abstract thinking that is required in the job, specifically proposing frameworks for what kind of information we need to either go and get or state as assumptions in order to support a decision. As the GP points out, the questions are testing how you model something you don't know but have common sense about, and significantly, how you don't get caught up in uncertainty or irrelevant minutia and identify some core assumptions.

That said, I agree 100% with you about "trick" questions. It seems some people cannot distinguish between "market sizing" (Fermi problems) where you need to make reasonable assumptions and describe a model of something, and "a man is found in scuba gear in the middle of the forest" brainteasers for kids. I have no idea why anyone asks brainteasers in an interview.

> I have no idea why anyone asks brainteasers in an interview.

I'll go one further - interviewers doing Myers Briggs Personality Tests.

This is really good insight and reminds me of similar industry norms that often get strong pushback (e.g. OKRs). Often there is solid reasoning behind why and when you'd want to employ these practices. However, enough organizations implement them just because they've seen them used or because there's hype around them. Eventually the poor implementation attracts pushback, but often the naysayers also don't understand the reasoning behind the practice.

I personally didn't understand what these types of questions would accomplish, but your explanation makes sense and it seems like this would be a good tool in the hiring toolkit.

> But "why are manhole covers round?" is not. It's a stupid "have you ever heard this clever fact" question that's often designed just to figure out if you're clever and/or lucky vs testing actual knowledge and skill.

I once had this interview question, gave them the expected answer, and cited the source of that answer. I never did get that job.

I am going to take your claim a step further and assert that this type of question can only reliably test cleverness if the interviewee is properly primed (e.g. there was a prior discussion of workplace safety), then the important thing is explaining why. Otherwise the only way to arrive at the answer is to realize that construction/maintenance workplaces place a high priority on safety. That isn't the type of thing you're likely to think about during an interview for a programming position.

I do agree that logic puzzles which require a single observation and/or have a binary outcome tend to be very bad interview questions. That being said, I think the original post demonstrates that the issue with the manhole cover problem isn't necessarily the question itself, but how it's evaluated. I'd be quite pleased if I were asking the question and the interviewee gave that answer (though I might raise the point that the configuration necessary to drop a rectangular cover is also one of the most ergonomic for holding it). The general notion that there is a single right and wrong answer is incongruous with a class of question used to evaluate the candidate's reasoning ability. Though I wouldn't be surprised if these were historically evaluated appropriately and candidates simply misinferred why they were being rejected.

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