How good are you at calculating risk? - Gerd Gigerenzer

譯者: Clement Fu 審譯者: SF Huang

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

「新藥降低心臟病發風險四成」

Shark attacks are up by a factor of two.

「鯊魚襲擊事件翻倍」

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

「日飲一升汽水，罹癌機會倍增」

These are all examples of relative risk,

以上都是相對風險的例子

a common way risk is presented in news articles.

也是新聞內容中慣用的風險陳述方法

Risk evaluation is a complicated tangle of statistical thinking

風險評估之所以複雜難評

在於其考量了統計思維與個人偏好

and personal preference.

One common stumbling block is the difference between

剛才說的「相對風險」與「絕對風險」

relative risks like these and what are called absolute risks.

兩者的區別在哪裡，難倒不少人

Risk is the likelihood that an event will occur.

「風險」是將來會發生某件事的可能性

It can be expressed as either a percentage—

表達的方式可以是百分比

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

例如：11% 的 60 至 79 歲男性

between the ages of 60 and 79—

會心臟病發作

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

或是比率：每年在澳洲西岸的潛水者

每二百萬人就有一人 遭受鯊魚致命的攻擊

will suffer a fatal shark bite each year.

These numbers express the absolute risk of heart attacks

這兩個數字，分別代表兩個群體中

心臟病發與鯊魚襲擊的絕對風險

and shark attacks in these groups.

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

風險變化也可用相對或絕對數字表示

For example, a review in 2009 found that mammography screenings

例如：2009 年一項檢閱發現

乳房攝影篩檢可減少乳癌死亡個案

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

從每一千名女性中 5 宗降至 4 宗

The absolute risk reduction was about .1%.

絕對風險下降了約 0.1%

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

但癌症死亡個案由 5 宗減至 4 宗

相對風險就下降了 20%

is 20%.

Based on reports of this higher number,

引用了這個較高的數字

people overestimated the impact of screening.

就會讓人高估了篩檢的影響

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

為了理解兩種表達風險的方法 所造成的差別有多大

let’s consider the hypothetical example of a drug

可以看看一個假想範例：

that reduces heart attack risk by 40%.

一款新藥減少心臟病發的 風險為 40%

Imagine that out of a group of 1,000 people

在一個沒吃新藥的 1,000 人群體中

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

10 人會心臟病發

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

絕對風險是千分之十或 1 %

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

假設有相似的 1,000 人吃了新藥

the number of heart attacks would be six.

有 6 人心臟病發

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

換言之，服用新藥能避免 十分之四的人心臟病發作

a relative risk reduction of 40%.

相對風險下降 40%

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

此時，絕對風險僅從 1% 降至 0.6%

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

但減少四成的相對風險 聽起來卻非常顯著

Surely preventing even a handful of heart attacks,

心臟病發等劫數，一宗都嫌多

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

能避就避，總值得一試吧？

Not necessarily.

也不一定

The problem is that choices that reduce some risks

問題是選擇降低某些風險時

can put you in the path of others.

可能會讓你暴露在其他風險之下

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

假設新藥會導致 1% 的 病人中的半數罹癌

In our group of 1,000 people,

在我們那 1,000 人群體中

four heart attacks would be prevented by taking the drug,

吃新藥能避免 4 宗心臟病發

but there would be five new cases of cancer.

卻新增 5 宗罹癌個案

The relative reduction in heart attack risk sounds substantial

心臟病發的相對風險看似下降很多

and the absolute risk of cancer sounds small,

癌症的絕對風險也看似很小

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

但屈指一算，實際數字其實差不多

In real life,

在現實生活中

everyone’s individual evaluation of risk will vary

每個人的自我風險評估各異

depending on their personal circumstances.

取決於個人的狀況

If you know you have a family history of heart disease

如果你知道家族有心臟病史

you might be more strongly motivated to take a medication

你也許會有較強的動機去服藥

that would lower your heart-attack risk,

減少自己心臟病發的風險

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

就算明知絕對風險只下降一點點

Sometimes, we have to decide between exposing ourselves to risks

有時候，我們要在各種

無法直接比較的風險中做取捨

that aren’t directly comparable.

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

譬如，治心臟病的新藥

讓身體衰弱的機率提高，卻不會致命

of a debilitating, but not life-threatening,

side effect like migraines rather than cancer,

有偏頭痛的副作用，但不會致癌

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

這些都會影響到 我們考量承擔何種風險

And sometimes there isn’t necessarily a correct choice:

有時並無所謂的正確抉擇：

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

即使鯊魚襲擊的風險微乎極微 但有人能免則免

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

並不稀罕錯失一場海洋暢泳

while others wouldn’t even consider skipping a swim

但有人卻不會為了那微不足道的

to avoid an objectively tiny risk of shark attack.

鯊魚襲擊風險，而放棄游泳

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

因為上述理由，風險評估其實很棘手

and reporting on risk can be misleading,

風險報告亦可能誤導民眾

especially when it shares some numbers in absolute terms

尤其是當絕對數字與相對數字

and others in relative terms.

混雜呈現時

Understanding how these measures work

了解風險評估如何運作後

will help you cut through some of the confusion

能夠協助你釐清疑惑

and better evaluate risk.

將風險評估做得更好