As the story goes, the legendary marksman William Tell was forced into a cruel challenge by a corrupt lord.
就像故事里说的那样，传说中的神射手威廉·泰尔被一个腐败的地主强迫进行一个残忍的挑战。
William's son was to be executed unless William could shoot an apple off his head.
威廉的儿子会被处决，除非威廉可以从儿子头上把一个苹果射下来。
William succeeded, but let's imagine two variations on the tale.
威廉成功了，但是让我们来设想一下这个传说的两个版本。
In the first variation, the lord hires a bandit to steal William's trusty crossbow,
在第一个版本里，那个地主雇了一个流氓偷走了威廉信赖的弓，
so he is forced to borrow an inferior one from a peasant.
于是他不得不从一个农民那里借一把差一点的弓。
However, the borrowed crossbow isn't adjusted perfectly,
然而，这把借来的弓没有调整到位，
and William finds that his practice shots cluster in a tight spread beneath the bullseye.
并且威廉发现，他试发的箭都紧凑地集中在靶心的下方。
Fortunately, he has time to correct for it before it's too late.
所幸，他在为时已晚之前还来得及作出调整。
Variation two: William begins to doubt his skills in the long hours before the challenge and his hand develops a tremor.
第二个版本：在挑战之前很长一段时间，威廉开始怀疑他的技术，他的手开始发颤。
His practice shots still cluster around the apple but in a random pattern.
他试发的箭仍然簇集在苹果周围，但是以一种随机的形式。
Occasionally, he hits the apple, but with the wobble, there is no guarantee of a bullseye.
很偶然地，他射中了苹果，但是在这种不稳定的情况下，不能保证会射中靶心。
He must settle his nervous hand and restore the certainty in his aim to save his son.
他必须让紧张的手安定下来，并且重新恢复他瞄准的确定性来救他的儿子。
At the heart of these variations are two terms often used interchangeably: accuracy and precision.
这些变种的核心是两个经常交替使用的词：准确度和准确性。
The distinction between the two is actually critical for many scientific endeavours.
两者的区别对于科学事业来说，其实很关键。
Accuracy involves how close you come to the correct result.
准确度涉及到你和正确结果的接近程度。
Your accuracy improves with tools that are calibrated correctly and that you're well-trained on.
你的准确度可以随着工具的正确校准，以及你得到的良好的训练而提高。
Precision, on the other hand, is how consistently you can get that result using the same method.
另一方面，准确性，是你用同一种方法得到的结果能够有多一致。
Your precision improves with more finely incremented tools that require less estimation.
精度提高并且不需要太多估测的工具，可以提高你的准确性。
The story of the stolen crossbow was one of precision without accuracy.
偷弩弓的故事说的就是有准确性，无准确度。
William got the same wrong result each time he fired.
威廉每一次射箭都得到错误的结果。
The variation with the shaky hand was one of accuracy without precision.
手抖的版本是有准确度，无准确性。
William's bolts clustered around the correct result, but without certainty of a bullseye for any given shot.
威廉的弩箭集中在正确结果周围，但是对于任意一次射击都不能有把握射中靶心。
You can probably get away with low accuracy or low precision in everyday tasks.
在日常的任务里，低准确度或者低准确性或许影响不大。

But engineers and researchers often require accuracy on microscopic levels with a high certainty of being right every time.
但是工程师和科研人员常常需要显微镜级别的准确度，以及有很大把握每次都得到正确结果。
Factories and labs increase precision through better equipment and more detailed procedures.
工厂和实验室通过更优的设备和更细节的流程来提高准确性。
These improvements can be expensive, so managers must decide what the acceptable uncertainty for each project is.
这些提高可能很昂贵，所以管理者必须决定每个项目可以接受的不确定性是多少。
However, investments in precision can take us beyond what was previously possible, even as far as Mars.
然而，对于准确性的投资，可以让我们超越之前的可能，哪怕是在像火星这么远的地方。
It may surprise you that NASA does not know exactly where their probes are going to touch down on another planet.
可能让你吃惊的是，美国国家航空局也不知道具体他们的探测器会在另一个星球哪里着陆。
Predicting where they will land requires extensive calculations fed by measurements that don't always have a precise answer.
预测着陆地点需要大量的计算和测量，而这些测算并不总是有一个准确的答案。
How does the Martian atmosphere's density change at different elevations?
火星大气密度在不同海拔会如何变化？
What angle will the probe hit the atmosphere at?
探测器会以什么角度接触大气层？
What will be the speed of the probe upon entry?
探测器进入的速度会是什么？
Computer simulators run thousands of different landing scenarios, mixing and matching values for all of the variables.
电脑模拟器运行了上千种不同的着陆方案，混合搭配所有变量的值。
Weighing all the possibilities, the computer spits out the potential area of impact in the form of a landing ellipse.
权衡过所有的可能性，电脑会计算出一个可能的接触区域，一个椭圆形的着陆区。
In 1976, the landing ellipse for the Mars Viking Lander was 62 x 174 miles, nearly the area of New Jersey.
在1976年，海盗号着陆器的椭圆着陆区域是62×174英里，差不多是一个新泽西州的大小。
With such a limitation, NASA had to ignore many interesting but risky landing areas.
在这种局限下，美国航天局只能忽略很多有趣但是危险的着陆地带。
Since then, new information about the Martian atmosphere, improved spacecraft technology,
从那时起，火星大气的新资料提高了宇宙飞船技术，
and more powerful computer simulations have drastically reduced uncertainty.
而更强的电脑模拟技术也大大减少了不确定性。
In 2012, the landing ellipse for the Curiosity Lander was only 4 miles wide by 12 miles long,
在2012年，好奇号的椭圆着陆区仅有4英里宽，12英里长，
an area more than 200 times smaller than Viking's.
比海盗号的小两百多倍。
This allowed NASA to target a specific spot in Gale Crater,
这让美国航天局得以瞄准盖尔陨坑里一个特定的点，
a previously un-landable area of high scientific interest.
一个之前无法着陆却有极高科学价值的地区。
While we ultimately strive for accuracy, precision reflects our certainty of reliably achieving it.
我们在无止境地追求准确度，而准确性反映了我们有把握得到准确度的确定程度。
With these two principles in mind, we can shoot for the stars and be confident of hitting them every time.
牢记这两个原则，我们可以射向星星，并且相信每一次都可以将他们击中。