rule of thumb 凭经验 凭感觉

rule of thumb 凭经验 凭感觉 字面的意思是拇指规则，实际上是指凭经验的方法，粗略不精确但很方便、实用的方法。
例如：

As a rough rule of thumb, for every 1 percent of gross national product devoted to military spending, overall economic growth is reduced by about one-half a percent.
粗略的计算，每当国民生产总值的1%贡献给军费开支时就会使得整个经济增长下降0.5%。

这个词语的英文解释是：
A means of estimation made according to a rough and ready practical rule, not based on science or exact measurement.
上面的意思是说：一种根据马马虎虎的实践规则估算的方法，不是基于科学或者精确的测量。

这里用到一个词 rough and ready ，很有意思，将在稍后介绍。

这个词实际还有非常用意思的起源，在这里介绍一下。主要有两说：

一说是木匠们经常用他们的大拇指而不是尺子来测量东西，这种说法倒是和它现在的意思（一种不精确，但是可信、方便的标准）吻合。

还有一说是这个短语与一部法律有关，这个法律限定了丈夫用来殴打妻子的木棒的粗度不能够超过他的大拇指。但是这说法被当作一个恶作剧而不被人相信。据传，这是一个名为Sir Francis Buller的英国法官宣布了这条法律。但是Buller是否真的宣布了这条法律，是很有疑问的，也没有证据表明是他宣布了这条法律。事实上，这条法律是如此的不受到公众的欢迎，以至于这位法官在一副讽刺的漫画中被称之为Judge Thumb(拇指法官)。这篇文章右侧的插图也就是当时的这幅漫画。画中一个丈夫正在鞭笞他的妻子，而一位法官为他送来了两捆拇指粗细的棒子。而这幅漫画的名字就《拇指棍，家庭暴力专用，保证合法》

以下是我摘抄自维基百科中关于the rule of thumb的来源的解释，有兴趣可以读一下。
The term is thought to originate with wood workers who used the length of their thumbs rather than rulers for measuring things, cementing its modern use as an inaccurate, but reliable and convenient standard.

It is often claimed that the term originally referred to a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife, but this has been fully discredited as a hoax

Sir Francis Buller, a British judge, was alleged to have stated that a man may
legally beat his wife, provided that he used a stick no thicker than his thumb. However, it is questionable whether Buller ever made such a pronouncement and there is even less evidence that he phrased it as a "rule of thumb"; the rumoured statement was so unpopular that it caused him to be lambasted as "Judge Thumb" in a satirical James Gillray cartoon.

最后来说一下这个英语短语的几个例句：
1.
I haven't been taught the finer points of carpentry; I just make things by rule of thumb.
我从来没有被教过木工，只是按自己的经验来。

2.After I graduated, and joined international company, I started to use English in my e-mails within the company. It has been a rule of thumb that all international companies uses English as internal communication tool. English skill has been an important factor during interview process.
从我毕业后进入这家国际化的公司，我开始在公司的内部邮件中使用英语。现在已经成了一条不成文的规定，所有的国际化公司都会使用英语作为交流工具，而英语也在面试中的一个重要考虑的因素。

3.
As a rule of thumb , you should cook a chicken for 20 minutes for each pound that it weighs.
凭经验，每磅鸡肉你应该煮20分钟。