How good are you at calculating risk? - Gerd Gigerenzer

翻译人员: Yizhuo He 校对人员: Thomas Tam

A new drug reduces the risk of heart attacks by 40%.

一种新药可以降低 40% 患心脏病的概率。

Shark attacks are up by a factor of two.

鲨鱼袭击人类的概率翻了两倍。

Drinking a liter of soda per day doubles your chance of developing cancer.

每天喝一公升汽水会使 你患癌症的概率倍增。

These are all examples of relative risk,

这些都是相对风险的例子，

a common way risk is presented in news articles.

也是风险通常出现在 新闻报道中的方式。

Risk evaluation is a complicated tangle of statistical thinking

风险评估的复杂在于 它混合了统计思考

and personal preference.

和个人偏好。

One common stumbling block is the difference between

使人摸不着头脑的是

relative risks like these and what are called absolute risks.

相对风险与绝对风险之间的区别。

Risk is the likelihood that an event will occur.

风险是指一个事件会发生的可能性。

It can be expressed as either a percentage—

它可以用百分比来表示——

for example, that heart attacks occur in 11% of men

比如，11%年龄在60至79岁的男性

between the ages of 60 and 79—

可能突发心脏病——

or as a rate— that one in two million divers along Australia’s western coast

风险也可用比率来表示—— 每年每两百万沿澳洲西岸潜水的人

will suffer a fatal shark bite each year.

就会有一个遇上致命的鲨鱼袭击。

These numbers express the absolute risk of heart attacks

这些数字表示的是患心脏病

and shark attacks in these groups.

和鲨鱼袭击的绝对风险。

Changes in risk can be expressed in relative or absolute terms.

风险的变化既可以用相对数值 也可以用绝对数值来表示。

For example, a review in 2009 found that mammography screenings

比如，一篇2009年的报道 发现乳房X光筛查

reduced the number of breast cancer deaths from five women in one thousand to four.

可将患乳腺癌的概率 从千分之五降低至千分之四。

The absolute risk reduction was about .1%.

绝对风险降低了大约0.1%。

But the relative risk reduction from 5 cases of cancer mortality to four

而癌症死亡率从5例降至4例， 相对风险降低了

is 20%.

20%。

Based on reports of this higher number,

基于对较高的数字的报道，

people overestimated the impact of screening.

会导致人们高估X光筛查的影响。

To see why the difference between the two ways of expressing risk matters,

为了说明为什么 两种风险表示形式的区别很重要，

let’s consider the hypothetical example of a drug

让我们假设有一种药

that reduces heart attack risk by 40%.

可以降低40%患心脏病的风险。

想象有一千多个没有服用新药的人，

Imagine that out of a group of 1,000 people

who didn’t take the new drug, 10 would have heart attacks.

他们中有十个会犯心脏病。

The absolute risk is 10 out of 1,000, or 1%.

绝对风险就是千分之十， 或百分之一。

If a similar group of 1,000 people did take the drug,

假设有类似的一千多人服用了新药，

the number of heart attacks would be six.

那么犯心脏病的人数就减少至6。

In other words, the drug could prevent four out of ten heart attacks—

换言之，这种药可以避免 十分之四的人犯心脏病——

a relative risk reduction of 40%.

相当于降低了40%的相对风险。

Meanwhile, the absolute risk only dropped from 1% to 0.6%—

与此同时，绝对风险 只从1%降低至0.6%——

but the 40% relative risk decrease sounds a lot more significant.

但是降低了40%的相对风险 听上去效果更显著。

Surely preventing even a handful of heart attacks,

哪怕只能让很少的一部分人 免于患上心脏病，

or any other negative outcome, is worthwhile— isn’t it?

即使药物有其他副作用， 都是值得尝试的，不是吗？

Not necessarily.

这也不一定。

The problem is that choices that reduce some risks

问题在于当某些风险被降低时，

can put you in the path of others.

另一些风险会相对上升。

Suppose the heart-attack drug caused cancer in one half of 1% of patients.

假设预防心脏病的药物 会导致0.5%的人得癌症。

In our group of 1,000 people,

那么在我们举例的一千人中，

four heart attacks would be prevented by taking the drug,

服用此药能避免4个人犯心脏病，

but there would be five new cases of cancer.

但却会导致五个人患癌症。

The relative reduction in heart attack risk sounds substantial

相对下降的心脏病 患病风险听上去很显著，

and the absolute risk of cancer sounds small,

患癌症的绝对风险听上去也很小，

but they work out to about the same number of cases.

但它们的结果却是类似的。

In real life,

在现实生活中，

everyone’s individual evaluation of risk will vary

每个人对风险的评估

depending on their personal circumstances.

都会因个人境遇而不同。

If you know you have a family history of heart disease

如果你知道自己有 心脏病的家族史，

you might be more strongly motivated to take a medication

那么你可能更倾向于服用

that would lower your heart-attack risk,

能降低心脏病患病风险的药物，

even knowing it provided only a small reduction in absolute risk.

尽管你知道它只能降低 很少一部分的绝对风险。

Sometimes, we have to decide between exposing ourselves to risks

有时，我们做出的选择 会让我们面临不同风险，

that aren’t directly comparable.

但是这些风险 并不能相提并论。

If, for example, the heart attack drug carried a higher risk

假如，这个预防心脏病的药物

of a debilitating, but not life-threatening,

只会产生使人虚弱， 或是偏头痛等副作用，

side effect like migraines rather than cancer,

并非可致命的癌症，

our evaluation of whether that risk is worth taking might change.

那我们对此药的风险评估 就会产生变化。

And sometimes there isn’t necessarily a correct choice:

有时，绝对正确的选择 并不一定存在：

有些人可能会觉得就算 被鲨鱼袭击的风险极小，都应尽量避免，

some might say even a minuscule risk of shark attack is worth avoiding,

because all you’d miss out on is an ocean swim,

毕竟你只是错过一次 在海里游泳的机会而已，

while others wouldn’t even consider skipping a swim

但其他人就甚至不考虑

to avoid an objectively tiny risk of shark attack.

因为被鲨鱼袭击的 极小几率而错过游泳。

For all these reasons, risk evaluation is tricky at baseline,

由于以上原因， 风险评估通常都比较棘手，

and reporting on risk can be misleading,

同时风险报告也会有误导作用，

especially when it shares some numbers in absolute terms

尤其使用绝对值表示一些数字，

and others in relative terms.

用相对值表示另一些数字时。

Understanding how these measures work

理解风险评估的原理

will help you cut through some of the confusion

会帮助你理清头绪，

and better evaluate risk.

更好的评估风险。