# “Rough subitising / estimation” for better intuition and ability to apply arithmetic

tl;dr: Why do so many students have poor intuition of numbers, and what can be done about it?



I've always been good with numbers. As a maths tutor, one of the things I notice is how poor the intuition and techniques that some of my students have when it comes to approximating numbers (amount of things) and fractions of things.

Example

Quickly approximate how many lines are on the following page, just by looking at it:

When I ask this question to my students, they will either count the number of lines "One, two, three, four...", which takes ages and moreover is tedious, or will just stare at the page and guess a random number. Some will guess like "seventeen" or "fifty", or "no idea", which worries me, as it shows their subitising skills aren't up to par. That said, not all are bad at this and some students will make a reasonable approximation of twenty to forty.

One method I use is to approximate $$10$$ lines by: Scrolling my eyes from line $$1$$ to line $$3$$ while counting "One, two, three" in my head. Then keep counting in my head at the same speed, scrolling my eyes at the same speed, and then stop scrolling my eyes once I get to the number $$10$$ in my head. This will take all of about $$2$$ to $$3$$ seconds for me to do. Now I know that I won't have counted exactly $$10$$ lines, but that's besides the point: I'm trying to get an approximation/estimation.

Once I have this "length of $$10$$", I'll then make copies of the "length of $$10$$ lines", using my imagination. Basically, doing this in my mind's eye:

Although in reality what I think I do is I try to look at a line in line the bottom of the first arrow, then look at a line in line with the bottom of the second arrow, then look at a line in line with the bottom of the third arrow, and notice that the bottom of the third arrow is very near the line closet to the bottom of the page.

This technique of "close-ish" approximation is useful because now I can try and do things with the information I have, without having spent $$30$$ seconds figuring out how many lines there actually are, which I don't care about. I can do the above process very quickly (in less than $$5$$ seconds) due to using it so often. This shortcut saves lots of time and therefore I think if students did this, they would lose less focus in maths because using these estimations help get close to the right answer quickly. They should use these shortcuts when approximating to see if their answers make sense.

Now let's say I'm writing an essay with the above paper and I know I can write on average $$8$$ words per line. The teacher requires the essay be minimum 600 words. I have 10 pages left in my paper pad. Do I have to buy a new one on the way home from school or not? Now a lot of my students will struggle with this question, spend $$5-10$$ minutes on it, as if it's a really difficult problem-solving exercise, and then some will give up, some will give the wrong answer, and some will give the right answer, assuming the right answer is "yes there is enough paper left because $$8\times 30\times 10 = 2400 > 600.$$ "

But here is my thought process: $$8\times 30\times 10 = 2400,\$$ so you only need $$\frac{600}{2400} = \frac{1}{4}$$ of your $$10$$ pages (i.e. $$2\frac{1}{2}$$ pages), but since you're going to be making notes and drafts on your essay, you might need to use $$7$$ or $$8$$ pages total. Plus there's the other homework and I need more paper for school tomorrow and I don't want to buy it from the shop tomorrow morning so, hmmm... so yes I really do need to buy another book pad now. If I had $$30$$ pages rather than $$10$$ pages left in my pad, I would then think, "hmmm... how much paper do I need for homework tonight? Maybe like 15 pages total. And I'm pretty sure I can get away with $$30-15=15$$ pages at school tomorrow, so I'm not going to bother going to the stationary shop after school today".

So I think it's this fluent usage of maths within a self-conversation that improves my intuition. It is realistic that I would have actually thought these things on the way home from school, whereas I imagine students give up on this thought process and just think, "I'm going to get a new pad from the stationary shop now", or "I'll borrow paper from James at school tomorrow", or most likely they won't think about it at all. In order to have a thought process similar to my one, one must have enough confidence in their own thought process to begin with.

But I think it's exactly this confident application of arithmetic in self-conversations assisting in everyday life things that helped people including myself get really good at doing arithmetic quickly and well. So we maybe have a kind of a chicken-and-egg thing: students won't do this unless they have confidence, but they won't ever be confident unless they start doing it.

Now, my thought processes outlined above will not be a revelation to anyone here (I would hope). But I think lots of students don't use this and other similar methods to estimation-check their working, or solve everyday mini-problems. So I guess my over-arching questions are:

Q1) what can be done to help students improve these important quick estimation skills?

Q2) How can we be encouraging students who are not naturally good at maths and arithmetic and fractions to do this on a day-to-day basis as part of their self-conversation? Or is it unethical or unrealistic to expect to be able to influence their self-conversation?

Originally my question also included "estimating fractions" (which is closely related to this question) to help improve intuition with fractions, but this question is already overly-long and a ramble (apologies for that), so maybe I will ask that as a separate question in the future.

Possible answer to my own question: I need to convey to my students the methods to help improve intuition of numbers outlined above (and various other methods) in a clear way. I think I already do this though. But maybe I need to do it more slowly and clearly? I'm not sure.

• The book Math Semantics, by Edward MacNeal, addresses this question at length in a chapter on estimation. He talks about your web of knowledge (some landmark numbers it helps to know, like the population of the world and the U.S.), among other things. Reading it will give you some good ideas. bookshop.org/books/mathsemantics-making-numbers-talk-sense/… – Sue VanHattum Apr 30 at 22:14
• The book by Bentley, "Programming Pearls" (2nd edition 1999, Addison-Wesley) has a chapter on "back of the envelope" estimates. Quite enlightening. And yes, the phenomenon you note is quite widespread. – vonbrand May 2 at 3:51