"The suspect is 1,828,800 microns tall, and his irises reflect 465-nanometre light..."

A reader wrote to tell me that he'd replicated my ice-resistance-measuring experiment, with the same results - about ten million ohms per inch. Then he said:

...although in Oz, shouldn't that have been centimetres?

This pressed one of my numerous Talk Buttons, so I thought I'd pour my canned rant on this subject out into a blog post where you all have to put up with it, rather than only favouring that one correspondent with my deathless wisdom.

Because nobody's forcing me to stick to a style guide, I freely mix metric and imperial units - doing my best to avoid the traps that lie therein - when I think it's appropriate.

Fractions of inches are seldom useful for anything (to me), and are a pain to work with too - I've got a lovely little German Imperial-unit vernier caliper that confuses the heck out of me every time I try to use it. Metric vernier scales are easy, but the imperial one is another of those things that slither out of my brain as soon as I put the caliper down.

But metric units just don't come in the right sizes for some measurements. "About an inch", as in the ice-resistance measuring, clearly conveys the rough-eyeball-distance-measuring I was doing. The metric equivalent either suggests an excessive level of precision ("about 2.5cm" gives the impression that the range is no more than 2.3 to 2.7...), or is cumbersome ("between 2 and 3cm").

My favourite example of not-so-useful metrication is in measuring human height. Australian publications usually have a style guide that forbids feet and inches, or at least requires metric equivalents to be added in brackets. So "the suspect in the Brooklyn Slasher murders has been described as being about 6 feet tall" becomes "...about 183cm tall", which again suggests more precision than actually exists in the measurement.

Some people might even say "182.9cm" in this situation, giving the impression that someone's measured the suspect with a micrometer. Since a person's height can easily change by more than an inch depending on what shoes they're wearing and slight changes in posture, I think most human height measurements with precision beyond the inch level are actively misleading.

(Wikipedia has a good little article on "false precision". And here's a piece on seeing false precision where it in fact does not exist. I ramble on about the limits to precision in real-world measurement here.)

30 Responses to “"The suspect is 1,828,800 microns tall, and his irises reflect 465-nanometre light..."”

  1. Jono4174 Says:

    Re: Australian Publication Style Guide

    I remember reading in the West Australian newspaper about the improvements in DNA testing.

    "before, you needed a blood stain about the size of a quarter (25 cents) to identify a suspect"

    25 cents!!!!!!!

  2. iworm Says:

    I so want to disagree with you, and almost thought I could. I then realised that if anyone ever makes me say "A half litre of beer please guvnor" instead of "A pint please" I'd poke them in the eye. So I kinda know what you mean.

  3. evilspoons Says:

    For heights, one could say "about 180 cm" or, to really confuse everyone, "about 18 dm".

    Being born in the 80s in Canada, I don't know much in the way of imperial measurements (being an electrical engineer and running into the ass-backwards way most mechanical things are designed has started to change that) but even I use feet/inches for heights.

  4. peridot Says:

    What's wrong with "a couple of centimeters" and "about 1.8 m"? I mean, okay, you only get one size of standard error per decade this way, but inches and feet are no better (unless you want to use barleycorns or some other hilarious unit.

    Though, I have to say, I have no stones to throw - in astronomy we use any number of horrible units - the Eddington limit for a one solar mass object is about 10^38 ergs per second, a neutron star at a kiloparsec is pretty close, right ascensions are measured in hours minutes and seconds - which are different from the degrees arcminutes and arcseconds (and milliarcseconds) we use for declination - and don't get me started on the magnitude system. Magnetic fields are measured in Gauss, which I can live with, but most astronomers don't even seem to know the units of voltage that they use!

  5. Malcolm Says:

    Dan, you're obviously the product of a substandard education, or perhaps subversive familial Imperialist indoctrination.

    Apart from 12 months of primary school in 1980 when the Americans had their chance with me, my schooling was refreshingly metrified. I now rarely stray except when humouring USians.

    I'd say your eyeballed distance was close enough to 2cm. And I know my own height (173cm) but everyone else is bloody well near enough to 2m (one sig fig, all the way).

    On second thoughts, maybe witnessing the feeble, tail-end-of-the-Carter-administration US attempts at metrification cemented my need to be brave and true to SI units. We all need something to believe in, after all!

  6. m0rm3gil Says:

    One area where imperial units always get used and that I can never wrap my head around is weights of babies.

    Nobody else expresses their weight in pounds these days but if you're a neonate then the metric system seems to go out the window.

    And Jono - a few years back the Australian federal govt released a suicide information website. It said people often kill themselves in winter, especially around Xmas time. Lovely to think of all those tax dollars going to someone who can cut and paste...

  7. DBT Says:

    One might also reference the Gimli Glider which demonstrates this hazard as applied to refuelling a Boeing 767.

  8. PG Says:

    It's sad to see a (mostly) reasonable human being inventing reasons to justify his weird behavior :) "Eyeballing" an inch is not really easier than naming a reasonable number of [whole] centimeters - and the precision is about the same either way.

    The same goes to heights - I can't eyeball them with better precision than 10cm (not 31cm, by the way), so "six feet two inches" feels quite weird: first you have to (quite precisely) imagine six feet, and then add two inches...

  9. Darien Says:

    For what it's worth, Dan, I'm completely on your side. I have no problems using whichever unit is more convenient or appropriate for me in any given situation, and not treating systems of measurement like they're some amazing political cause I need to be supporting.

  10. Bern Says:

    evilspoons: the reason most mechanical stuff seems designed ass-backwards to you is that in Canada it's probably mostly designed in that bastion of imperialism known as the USA. :-P

    And while I generally can live with a bit of imperial measure, one thing that really gets to me lately is the way Bunnings, one of the biggest hardware chains in Australia, an almost exclusively metric country, seem to stock more stuff in imperial sizes than metric.

    At least they *do* stock metric. Just usually not in the product line you're after... :-/

  11. rocketfire Says:

    I do like the idea of a making an inch (25mm) and a foot (300mm) unless specifically preceded with term "imperial", it's the best of both worlds.

  12. kamikrae-z Says:

    @10 - Screw threads are a whole different ball game!

    I find the easiest way to convert is to simply carry around a cheapo measuring tape with inches on one side and meters on the other.

    Having said that, brass plumbing fittings are still imperial, but often labelled as their metric equivalent (eg. 25.4mm).

  13. lummox Says:

    Using metric units to measure people or slabs of ice really isn't much of a problem if you're used to it. You just measure to the closest reasonable accuracy. We in the metric-only countries don't of course say "He's about 183 cm tall." We say "He's about one eighty."

    Your slab of ice would be "about 2 cm thick" or "about 3 cm thick" which in the realm of eyeball measurements amounts to pretty much the same thing.

    Oh and we don't order half a litre of beer, we order a beer :-)

  14. fizz Says:

    The problems you pointed out only come out when you try converting from imperial to metrics. Really, you're having troubles only because deep down you're still thinking imperial, while metrics are an acquired habit... Growing up in a country where imperial are used exclusively for some special products (floppy disks and monitor sizes) I can guarantee you that the problems you mention are totally absolutely not existing, like other people already said.
    About size of beers and other drinks, there's also a standard about small, medium, large glasses and so on, corresponding to 0,25 0,33 and 0,5 litres. So, at a pub I ask for a medium beer... :p

  15. NickL Says:

    I'm from the US. I was born and raised here. Aside from a two week vacation to New Zealand in 1994, I've never been out of the country.

    I think the problem most people here have with the metric system is a stunning lack of reference points. And XKCD comic addresses this nicely: http://xkcd.com/526/
    People here can intrinsically draw a foot with reasonably precision, but not a centimeter. People understand what a pound weighs, and what it feels like to pick up a 70 pound bag of salt, but have no idea what 31 Kilograms would look/feel like.

    Furthermore, fractions suggesting any degree of precision simply aren't used much here. Fractions with a denominator larger then 4 aren't used by common people in daily life. (In common speech, you might encounter 'a half' or 'a quarter') People working in building construction often measure down to a 16th, but that are surprisingly few other examples where that happens.

    In the engineering and manufacturing trades, almost everything is measured in inches. For granularity under an inch, instead of using fractions, we use tried and true, power of 10 decimal places. For instance, a typical dimension on a print might read: 27.210 +- 0.015. The ‘default’ in most prints I’ve read is specific to one thousandth of an inch. Tolerances are explicitly given, so no precision is inferred from the number of decimal places.

    This is reflected in many other places as well. Buying gasoline (petrol?) for your car? It will be measured in gallons, to three decimal places.

    In my experience, fractions simply aren’t used that much here.

    For an even more exciting topic, let's talk about threaded fasteners, wire gauge sizes, and 'letter' drill bit sizes. :-P

  16. Ice8205 Says:

    It's all well and fine to mix units, until you lose a space ship over Mars because someone converted the units wrong.


  17. Chris L Says:

    In Canada we measure ourselves in feet, inches & pounds, even though our driver's licenses have used centimetres and kilograms for 30 years. We just never gave up on that one.

  18. Chris L Says:

    We also never stopped selling things by the pound, we just changed the label to 454 g.

  19. MrWorf Says:

    NickL: Interesting comment... Let me just put things in perspective.

    I was born and raised in Sweden, I never had to deal with the imperial measurement system until I started working in the US for about a year ago. If someone says to me that a person was 6 feet tall, then I really don't have a clear idea as to how tall that person really is. Neither does gallon or ounces give me any idea of volume. Don't get me started on pounds :)

    The point being, we find the things we are used to a lot easier to deal with than something we're not using.

    Making assumptions that just because you can correlate a foot to something physical still doesn't make things easier if you've been using metric the entire life (or vice versa).

    Oh, and while I have the microphone, let me just mention that the metric has one advantage (IMHO) which is that it is based on a decimal system (millimeters, centimeters, liter) making it easier to convert on the fly :D

    And for the record, forcing my or your view of how to measure stuff on anyone else isn't good. But it would help if we all had the same system ;)

    I'm learning to deal with imperial units, but they still give me headaches.

  20. TwoHedWlf Says:

    And what about people's weight in Stone? Gaaahhh! As an american living in NZ, I hate imperial. I refuse to use it for anything requiring more precision than, "I was going about 100 mph hour when the cop saw me."

  21. matkun Says:

    NickL: To the contrary, I don't know how much an inch is but I know a centimeter is roughly the width of a fingernail (non-thumb) and that 10 centimeters is about the length of my hand.

    I know how much a kilogram weighs in my hands but have trouble feeling out how many pounds something weighs.

    It all depends on what you grew up with and made correlations with. It's not like those correlations are at all accurate anyways. ie: a petite woman's foot compared to a basketball players.. They are both a foot long, right?

  22. Thuli Says:

    At this point I must draw everyone's attention to Frink!

  23. j Says:

    Metric superheroes:

    Mixing imperial and metric is something that I do naturally too - but I wouldn't ever bother justifying it. We live in a period of time where the US refuses to change and the rest of the world's moved on, so it should be expected that we get stuck with a mix of both for a while.

  24. MorganGT Says:

    I tend to use whichever measurement system is appropriate to the situation, and just mix and match freely if it suits. Because I'm into old cars, a lot of my garage tinkering involves Imperial fractions, but I am also into old Japanese motorbikes, so Metric raises its head there. And I work in a manufacturing environment where everything is measured in Imperial decimals, usually down to 0.001", or 0.0001" in some specific circumstances. In that situation the unit (inch or centimetre) becomes irrelevant, the fact we work in decimals rather than fractions is the key to a practical and workable metrology environment.

    Most of our equipment is fabricated in-house for specific jobs, so it gets built using adapted components designed for something else, which can often mean a mix of units throughout - Imperial fraction, Imperial decimal, Metric, plus the often used exact measurement of 'grind it until it just fits'. Sometimes even a secondhand Whitworth bolt will be pressed into service.

  25. kamikrae-z Says:

    @14 - Now there's something I've never understood - using commas as decimal points. I do like using commas as thousands separators although I believe the correct standard is to use a space instead.

  26. NickL Says:

    @matkun & @MrWorf

    Thanks for the input. I certainly agree! It's really a matter of the reference points one acquires when growing up.

    Honestly, I think a measurement system based on powers of 10 makes far more since then the imperial. (Wait, 4 or 8 pints in a gallon....) That said; all of my machine tools are graduated in inches, as are my measuring instruments.

    While this is reflected throughout industry, things are changing. With the advent of CNC equipment, changing from metric to imperial is just one line of G code. Some manufacturing firms are already completely metric. (For instance, a large Honda Motor Company plant that makes engines in Ohio, USA)

    We may switch over entirely someday - most food products are now labeled in both imperial and metric.

  27. FuzzyPlushroom Says:

    "Now there’s something I’ve never understood - using commas as decimal points."

    Honestly, the reverse bothers me just as much - 14.000 isn't fourteen large, it's fourteen and no thousandths, dammit!

    Also, "stone" should be outlawed. Seriously, what a waste of time. While I'm at it, decimeters need to be more common.

  28. derrida derider Says:

    Try looking at the sizes written on the sidewalls of your car tyres for confusion.

    The first number is the width of the tyre in millimetres. The next number is the rim diameter - in inches(!). Finally you are expected to work out the size in millimetres between the rim and the ground by multiplying the first number by the third number (the aspect ratio) and dividing by 100.

  29. girtby.net Says:

    Precisely Wrong

    Dan reminds me of a story I heard on an ancient [Media Watch](http://en.wikipedia.org/wiki/MediaWatch(TV_program) episode. It’s Stuart Littlemore-era Media Watch, and is sadly not in the otherwise extensive ABC online archives. hence you’ll...

  30. Matt Says:

    When I moved here to Germany, it took a while to get used to metric measurements being used for things like height. But as other people have mentioned, you don't tend to give your height to the cm, you say "about 190".

    > "A half litre of beer please guvnor"

    Yep, you often order "eine halbe" - a half (litre).

    Interestingly, there are still some units where Germany doesn't use the metric system - bar for pressure, beaufort for windspeed, (kilo)calories for food...

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