[LINK] Humans are bad at summing up a bunch of small numbers

post by Vladimir_Golovin · 2010-10-06T13:01:58.192Z · LW · GW · Legacy · 4 comments

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  "Outsmart your brain by knowing when you are wrong":http://troysimpson.co/outsmart-your-brain-by-knowing-when-you-are-w
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"Outsmart your brain by knowing when you are wrong":
http://troysimpson.co/outsmart-your-brain-by-knowing-when-you-are-w

Humans are incredibly bad at summing up a bunch of small numbers.  I had recently read a study that looked into why people are so bad at this task, but the important part was people commonly underestimate the total.

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Knowing what you are bad can be incredibly important.  Use this trick when estimating the timeline for a lot of small tasks, or figuring out what your monthly expenses are.   It always seems shocking your credit card bill is so high when it is a bunch of small purchases.  Learn what else your brain cannot perform well and use that to your advantage.

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comment by Relsqui · 2010-10-07T03:11:44.106Z · LW(p) · GW(p)

That's cute. I'm amused that when I stopped to make my five-second guess, I thought to myself, "it looks like 250-300, but I know it must be more, so let's say 400." Then I read on, saw the author had done the exact same thing, and thought, "oh, must be even bigger then." Yup.

For credit cards or shopping on a budget, I make a point of rounding up to the next dollar as I go, so as to consciously overestimate the total cost so far.

I suspect that this phenomenon arises because humans file 100 as "a really big number," and 20 as "a reasonable/moderate number," especially when thinking about the cost of purchases. It doesn't "feel" quite right that 100 is only five 20s!

Or, an example that still boggles me: $100 is only two new big-name video games on any of the most recent generation of consoles.

Replies from: erratio
comment by erratio · 2010-10-07T03:39:46.560Z · LW(p) · GW(p)

$100 is only two new big-name video games on any of the most recent generation of consoles.

In Australia it's only one, sometimes not even that.

comment by ShardPhoenix · 2010-10-06T23:58:03.387Z · LW(p) · GW(p)

I've noticed this when looking at cricket scores. At first glance, the total innings score often seems higher than it "should" given the individual batsmens' scores.

comment by JoshuaZ · 2010-10-06T14:40:39.426Z · LW(p) · GW(p)

Note that this happens with multiplication also. There's a classic study of anchoring bias where people are asked to estimate about how large 1*2*3*4*5*6*7*8 is or are asked to estimate how large 8*7*6*5*4*3*2*1. People asked either underestimate on average and underestimate the first much more than the second.

This makes me suspect that there's a general tendency to underestimate independent of what operations are being performed.

ETA: Some of this is discussed (with citations) in a previous post by Eliezer here.