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  • As the story goes, the legendary marksman William Tell

  • was forced into a cruel challenge by a corrupt lord.

  • William's son was to be executed

  • unless William could shoot an apple off his head.

  • William succeeded, but let's imagine two variations on the tale.

  • In the first variation,

  • the lord hires a bandit to steal William's trusty crossbow,

  • so he is forced to borrow an inferior one from a peasant.

  • However, the borrowed crossbow isn't adjusted perfectly,

  • and William finds that his practice shots

  • cluster in a tight spread beneath the bullseye.

  • Fortunately, he has time to correct for it before it's too late.

  • Variation two:

  • William begins to doubt his skills in the long hours before the challenge

  • and his hand develops a tremor.

  • His practice shots still cluster around the apple

  • but in a random pattern.

  • Occasionally, he hits the apple,

  • but with the wobble, there is no guarantee of a bullseye.

  • He must settle his nervous hand

  • and restore the certainty in his aim to save his son.

  • At the heart of these variations are two terms often used interchangeably:

  • accuracy and precision.

  • The distinction between the two

  • is actually critical for many scientific endeavours.

  • Accuracy involves how close you come to the correct result.

  • Your accuracy improves with tools that are calibrated correctly

  • and that you're well-trained on.

  • Precision, on the other hand,

  • is how consistently you can get that result using the same method.

  • Your precision improves with more finely incremented tools that require less estimation.

  • The story of the stolen crossbow was one of precision without accuracy.

  • William got the same wrong result each time he fired.

  • The variation with the shaky hand was one of accuracy without precision.

  • William's bolts clustered around the correct result,

  • but without certainty of a bullseye for any given shot.

  • You can probably get away with low accuracy

  • or low precision in everyday tasks.

  • But engineers and researchers often require accuracy

  • on microscopic levels with a high certainty of being right every time.

  • Factories and labs increase precision

  • through better equipment and more detailed procedures.

  • These improvements can be expensive, so managers must decide

  • what the acceptable uncertainty for each project is.

  • However, investments in precision

  • can take us beyond what was previously possible,

  • even as far as Mars.

  • It may surprise you that NASA does not know exactly where

  • their probes are going to touch down on another planet.

  • Predicting where they will land requires extensive calculations fed by measurements

  • that don't always have a precise answer.

  • How does the Martian atmosphere's density change at different elevations?

  • What angle will the probe hit the atmosphere at?

  • What will be the speed of the probe upon entry?

  • Computer simulators run thousands of different landing scenarios,

  • mixing and matching values for all of the variables.

  • Weighing all the possibilities,

  • the computer spits out the potential area of impact in the form of a landing ellipse.

  • In 1976, the landing ellipse for the Mars Viking Lander

  • was 62 x 174 miles, nearly the area of New Jersey.

  • With such a limitation,

  • NASA had to ignore many interesting but risky landing areas.

  • Since then, new information about the Martian atmosphere,

  • improved spacecraft technology,

  • and more powerful computer simulations have drastically reduced uncertainty.

  • In 2012, the landing ellipse for the Curiosity Lander

  • was only 4 miles wide by 12 miles long,

  • an area more than 200 times smaller than Viking's.

  • This allowed NASA to target a specific spot in Gale Crater,

  • a previously un-landable area of high scientific interest.

  • While we ultimately strive for accuracy,

  • precision reflects our certainty of reliably achieving it.

  • With these two principles in mind,

  • we can shoot for the stars

  • and be confident of hitting them every time.

As the story goes, the legendary marksman William Tell

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【TED-Ed】What's the difference between accuracy and precision? - Matt Anticole

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    稲葉白兎   に公開 2015 年 04 月 20 日
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