

Ask HN: real world examples of numbers with 3 decimal places - u296

sorry this is off-topic but there seem to be a lot of mathematicians here. I'm an elementary teacher and currently teaching children about decimal places and place value. I try to use real world examples to teach maths whenever possible but I can't think of any using 3 decimal places that would be relevant to such young children (aged 8-9).<p>any ideas?
======
tokenadult
_I'm an elementary teacher_

Definitely baseball statistics for that audience. Also finishing times for
sprints to thousandths of a second.

Not to answer your question, but to suggest useful resources for elementary
math teaching, I'll suggest

[http://www.amazon.com/Knowing-Teaching-Elementary-
Mathematic...](http://www.amazon.com/Knowing-Teaching-Elementary-Mathematics-
Understanding/dp/0805829091/)

an essential book for someone in your profession, and

[http://www.amazon.com/Vision-Elementary-Mathematics-W-
Sawyer...](http://www.amazon.com/Vision-Elementary-Mathematics-W-
Sawyer/dp/048642555X/)

a book with wonderful teaching tips, and

[http://www.amazon.com/Elementary-Mathematics-for-
Teachers/dp...](http://www.amazon.com/Elementary-Mathematics-for-
Teachers/dp/0974814008/)

a very excellent set of exercises for any elementary mathematics teacher
preparing lessons, and a great guide to the best available series of
mathematics textbooks in English.

You might also try asking your question on the appropriate forum

<http://www.artofproblemsolving.com/Forum/index.php?f=300>

(the forum is really for middle school math, but you could give it a try) on
the Art of Problem Solving site, a generally good site for discussion of math.

------
dxjones
How about measuring distance using the metric system. How tall are you? How
long is your hair? (in metres) How big is this room? How far is it to your
house?

They should be able to convert from being 127 cm tall to being 1.27 metres
tall, and then be able to run around measuring things that are a fraction of a
metre and multiple metres.

A good concept to add would be understanding things in multiples of ten. For
instance, once they know it is 1.5 kilometres (1500 m) from their house to the
school, they can use their understanding of decimals to tell them it would
take _roughly_ 1000 kids like them laid end-to-end on the sidewalk to cover
the distance from their house to their school. ... They know this just from
shifting the decimal point when comparing 1500 m to 1.27 m. If you are lucky,
they will even think about this lesson on their walk home!

( _roughly_ 1000 is a better estimate than 100 or 10,000 and in many
situations, that kind of estimate is good enough)

~~~
rkowalick
The problem with using the metric system in the US is that it really isn't
applicable to most real life situations and feels quite artificial. I recall
having that feeling as a child while being taught the metric system.

It's funny that they show you the superb benefits of the metric system very
early and when it is most apparant while doing basic arithmetic, yet you live
in a country where regular use of the metric just can't happen.

~~~
skwiddor
> it really isn't applicable to most real life situations

the irony is that as 1 liter of water weighs 1 kilogram estimates of various
things is made much easier.

I've often used that relation to my benefit.

~~~
ken
I don't disagree that the metric system is simpler, but that's maybe not the
best example, because the U.S. system has a similarly simple water volume-
weight equivalence, and it even has a catchy saying: "A pint's a pound, the
world around". :-)

------
niels_olson
Medicine: the pH of an arterial blood gas (ABG) sample is typically given out
to 4 digits. 7.380 is about normal. 7.211 might seem about the same as 7.219,
but if one sample is 7.211 and 15 minutes later it's 7.219, you're doing
something right. If it's 7.219 and in 15 minutes it goes down to 7.211,
something is very wrong.

Machine shops and engineering: every machinist makes every cut to, at a
minimum, the nearest thousandth of an inch, and often to the tenths of
thousandths; often over several inches (like piston heads and cylinders) This
is what slip-clutch micrometers are for.

A red optical filter for a physics experiment could easily be accurate to 720
nanometers with a band pass of 719.998 to 720.003 +/- 2 in the next digit.
There are lots of examples in physics, go get a physics supply catalog.

~~~
Zev
Awesome examples, but I would imagine that none of them would apply to someone
in elementary school (K-5, or children 10 and under, basically).

~~~
niels_olson
Trauma medicine, race car engines, and lasers. What kid under 10 wouldn't be
totally into those?

------
vaksel
stock prices gas prices(granted they use 9/10 as a fraction, but it can be
stretched)

p.s. you'll be happy to know that if you search google for "real world numbers
with 3 decimals" this thread is the #1 result

------
spooneybarger
baseball batting averages.

~~~
spooneybarger
for that matter, slugging percentage, on base percentage etc etc. many
baseball stats use 3 decimal places. baseball drew me into statistics when i
was that age.

~~~
sachinag
_Don't use slugging percentage_ \- it's not a percentage at all. Singles are
1, doubles are 2, triples are 3, and home runs are 4. It's a terribly named
statistic that's on its own bizarre scale.

~~~
spooneybarger
true, BUT... 3 digits are generally the norm and he original poster didn't
specifically mention that he wanted percentages. i would say slugging
percentage is another good example because you can go into the ideas of how it
is actually determined. the arbitrariness of it. leads into lots of other
wonderful esoteric basbeball stats, vorp and all the rest.

------
cromulent
1/8 = one slice of pizza

~~~
tokenadult
_1/8 = one slice of pizza_

How does this even answer the question that was asked?

(It's hardly likely that an elementary teacher would need this suggestion, as
all elementary math textbooks show fractions as slices of pies or pizzas.)

Do you mean because one-eight of a whole is 0.125 of a whole? That would be a
good reason to teach the decimal representation of that fraction, if that is
what you are suggesting.

P.S. Some pizza restaurants I frequent here cut pizzas into twelfths. This
probably depends on local custom and the size of the pizza.

~~~
cromulent
The question that was asked was: Any ideas?

That was my idea - a real-world example (for the age group) of why the third
decimal place is important.

Regarding your local pizzas, it was an example - it may not be valid for all
situations, unfortunately, but there's no dependency.

~~~
tokenadult
I'm surprised you didn't write 1/8 = 0.125 to make the point clear, because
1/8 of ANYTHING is 0.125 of the whole thing.

A cut of pizza into eighths can be done visually (and probably usually is done
visually) with no knowledge of decimals at all.

------
oomkiller
Fractions of time would probably be full of fail, since it uses a different
scale since it's time.

Try to think of an example using video games, maybe some RPGs use three
decimal places somewhere for stats.

Somehow as a civilization we must have decided somewhere that 2 fixed floating
point places was plenty for 99% of things, because you usually never see any
more than that unless you get into science.

Another possible way would be to show percentages, but that might also
complicate things if they don't know about them yet.

------
jlees
Not sure if this is how it works in the US but our petrol prices are often
technically 3 decimal places. 1.059 etc.

------
modoc
Interest rates (such as for mortgages) are often in three decimals e.g. 4.875%

------
izak30
Sales Tax in my city is 6.5% so as a decimal it hits the mark.

------
jraines
Lap times in F1/NASCAR?

------
ComputerGuru
temperature averages?

------
skwiddor
RDA's and food labelling

<http://en.wikipedia.org/wiki/Reference_Daily_Intake>

Then you can hide the fact they are doing math while teaching them about
healthy eating.

Incorporate things like <http://www.sugarstacks.com/> and
<http://www.wisegeek.com/what-does-200-calories-look-like.htm> and you can
have some interesting projects going

