# Why are estimation questions off-topic?

A recent attempt on my part to re-open an estimation question has failed, so I thought it might be worthwhile to continue the discussion on meta. The original question:

Estimate number of hairs on human head

This was marked as off-topic since physicists should not care about the number of hairs on a human head. I will resist the urge to answer that physics = everything, since then we will start getting questions about the economy on here...but I think that estimation questions should be allowed for three main reasons:

1) Every single physics textbook contains them. If chapter 1 of an introductory physics textbook is "off-topic", then how can we determine that anything is on-topic?

2) Physicists have a long tradition of estimation. The phrase "Back-of-the-envelope" originated in the hard sciences, and any googling on the issue will immediately tell you these kinds of problems are referred to as "Fermi problems" because he was so good at them (http://en.wikipedia.org/wiki/Fermi_problem). He used them to solve physical problems - like estimating the size of nuclear explosions.

3) They are a key part of the working physicists toolkit. During seminars/conferences, questions of the form "it seems like that value is too large" are important discussion points, and originate with such estimation techniques. They allow us to study the mechanisms behind physical phenomena and processes which we may not be able to understand from first principles ( i.e. the Drake equation). If someone is running a machine (accelerator, telescope, SEM, SQUID...) they will be required to make on-the-fly decisions based on estimations of physical parameters of the system.

So can someone tell me why these questions are not allowed? To me they are really characteristic of problems in physics. I hate using quotes to prove arguments, but I'm going to do so in this case:

"Never make a calculation until you know the answer: make an estimate before every calculation, try a simple physical argument (symmetry! invariance! conservation!) before every derivation, guess the answer to every puzzle."

                         Wheeler and Taylor, Spacetime Physics (1966).


## 4 Answers

I would add to the other answers that you seem to be taking the wrong principle away from this. Just because the question was about order-of-magnitude estimation, and it got closed, doesn't mean questions about estimation are off topic. So what you're asking here is not really a problem that needs to be solved: estimation questions are not off topic as far as I know.

However, no-effort homework-like questions in which someone asks us to solve a problem for them are off topic, and also questions which ask for an answer rather than about a physical concept are off topic. (Kind of the same thing.) This question about hairs on a head is precisely that type.

Let's assume the question, even in it's current form, were asked today. I have a feeling it would be closed under the homework policy because it shows no effort, doesn't ask about a physical concept, and essentially boils down to a copy-paste of the question asked.

So while I suppose one could argue that the close reason there is not the correct one, I still think it wouldn't be open anyway. I would vote to close it at least.

• It doesn't actually give a close reason on the question, as far as I can tell, which is probably an artifact of how the old closing system worked. But I agree; this strikes me as a fairly obvious case of a no-effort homework question. – David Z Dec 16 '13 at 23:36
• This clarifies the main point - the question was listed as off-topic, so I tried to fix that. Now I understand that wasn't the main problem with that question - I wish I had been informed about that from the beginning. – levitopher Dec 17 '13 at 2:20
• @levitopher At the time it was asked, "off-topic" was the reason. But the close system changed since then, as did our close reasons (which became much more specific). So at the time it was closed as "off-topic" but today it would be closed as "homework-like" that doesn't meet the policy standards to remain open. Subtle difference due to a system change in the time since that was asked. – tpg2114 Dec 17 '13 at 2:25

Multiple reasons why it should stay closed:

• You've made a very radical edit to it, completely changing what was asked. Ask it separately.
• It's not a practical Fermi problem as there is no starting ground. The classic piano tuner problem has a base of numbers that can be easily estimated.
• It's a tool, yes, but ultimately not a physics question. "How does one solve the Fermi problem of estimating the number of binary stars in an area" is an OK question. This is not. One could similarly reason that programming questions are on topic, since programming is a tool which is widely used in physics
• I would argue that keeping the current question is important because there are reasonable answers there already. That's why I elected to correct the current one. – levitopher Dec 17 '13 at 2:19

I reluctantly approved your radical edit to the question because I thought you put a lot of effort into it and I didn't entirely disagree with your main point.

The issue I see with the question as it is worded now is:

• About 80% of the question text is defending the general idea of estimation as a valid tool in physics

• The question itself (how many hairs on a human head) has almost no value to the community

I'm not against estimation questions but I think a question should be about the estimation technique or have some value to the overall community. If the goal is to have a good demonstration of estimation then we could have any question, not just human hair, be the stand-in-question that demos the technique.

I think this could be one of the cases when posting a question about estimation and then answering your own question would be a valuable contribution to the site.

• Ah... you know I was wondering about that edit. It kind of seemed too drastic to me but I wasn't sure. – David Z Dec 17 '13 at 0:14
• @DavidZ I went back and forth and considered just hitting "skip" to let others decide. I agree that it was very borderline. – Brandon Enright Dec 17 '13 at 0:15
• Ok, but you see the origin for the question in it's current form right? It was listed as 'closed: off-topic' not 'closed: homework violation'. I just tried to convince the community that the question was NOT off topic. At least I understand what some majority of people think about this - I was just misinformed. – levitopher Dec 17 '13 at 2:18