Shark attacks are up by a factor of two.

鯊魚攻擊事件增加了一倍。

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

每天喝一公升汽水罹癌率增加一倍。

These are all examples of relative risk,

這些都是相對風險的例子，

a common way risk is presented in news articles.

在新聞中很常見的風險報導方式。

Risk evaluation is a complicated tangle of statistical thinking

風險評估是一個牽涉了統計思維和個人偏好的

and personal preference.

複雜的糾結。

One common stumbling block is the difference between

一個最常見的阻礙就是

relative risks like these and what are called absolute risks.

相對風險和絕對風險間的差異。

Risk is the likelihood that an event will occur.

風險指的是事情發生的可能性。

It can be expressed as either a percentage—

風險可以用百分比來表示，

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

例如： 60-79 歲的男性中有 11%

between the ages of 60 and 79—

患有心臟病，

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

也可以用比率來表：每年澳洲西岸，每兩百萬潛水員中就有一人

will suffer a fatal shark bite each year.

遭受到鯊魚的致命攻擊。

These numbers express the absolute risk of heart attacks

這些數字表示了心臟病及

and shark attacks in these groups.

鯊魚攻擊的絕對風險。

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

風險的變化可以用相對或絕對的字眼來表示。

For example, a review in 2009 found that mammography screenings

舉例來說， 一項 2009 年的報告發現乳房攝影檢查

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

將乳癌死亡率從千分之五降至千分之四。

The absolute risk reduction was about .1%.

從絕對風險來說，是減少了 0.1% 。

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

但，對相對風險來說，從五例死亡降至四例，則是

is 20%.

減少了 20% 。

Based on reports of this higher number,

基於這個較高的數值，

people overestimated the impact of screening.

人們高估了乳房攝影的影響。

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

為了了解這兩種風險區別的重要性，

let's consider the hypothetical example of a drug

讓我們來假想有一種藥物

that reduces heart attack risk by 40%.

可以減少 40% 的心臟病風險。

Imagine that out of a group of 1,000 people

想像一下，在一組 1000 名沒有

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

服用新藥的人中，有十人會得心臟病。

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

絕對風險是千分之十或者可以說 1% 。

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

如果相似的一組 1000人有服用藥物，

the number of heart attacks would be six.

則得到心臟病的人數為六人。

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

換句話說，這種藥物可以預防十分之四的心臟病發率，

a relative risk reduction of 40%.

相對風險減少了 40% 。

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

與此同時，絕對風險僅僅是從 1% 降到了 0.6% 。

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

但降低了 40% 的相對風險聽起來顯赫多了。

Surely preventing even a handful of heart attacks,

既使僅能預防少數的心臟病發或者

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

其他負面作用，也值得了，不是嗎？

Not necessarily.

不盡然。

The problem is that choices that reduce some risks

問題在於，減少某些風險的選擇

can put you in the path of others.

可能讓你置身於其他風險。

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

假設心臟病藥物使得 0.5％ 的病人罹患癌症。

In our group of 1,000 people,

在千人小組當中，

four heart attacks would be prevented by taking the drug,

服用藥物可以降低四次心臟病發的機率，

but there would be five new cases of cancer.

然而卻會新增五例癌症。

The relative reduction in heart attack risk sounds substantial

心臟病的相對降低率聽起來很多，

and the absolute risk of cancer sounds small,

而癌症的絕對風險率聽起來很少。

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

但其實人數是相同的。

In real life,

現實生活中，

everyone's individual evaluation of risk will vary

每個人的風險評估都會因為

depending on their personal circumstances.

個人情況而有所不同。

If you know you have a family history of heart disease

如果你有心臟病家族史，

you might be more strongly motivated to take a medication

你可能會有更強烈的意願去服用

that would lower your heart-attack risk,

降低心臟病機率的藥物，

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

即使你知道僅能降低少少的絕對風險。

Sometimes, we have to decide between exposing ourselves to risks

有時，我們必須在不能比較的風險之間

that aren't directly comparable.

做出選擇。

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

假如，心臟病藥物帶有更高風險的

of a debilitating, but not life-threatening,

非致命性副作用，

side effect like migraines rather than cancer,

例如偏頭痛，而非癌症，

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

我們對於是否要冒這個風險的評估可能會改變。

And sometimes there isn't necessarily a correct choice:

有時候，這並沒有最正確的選擇，

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

有人認為，即使是最微小鯊魚攻擊風險也值得避開，

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

因為你僅是不能在海裡游泳而已，

while others wouldn't even consider skipping a swim

而另一些人則是不會為了避開

to avoid an objectively tiny risk of shark attack.

鯊魚攻擊的微小風險而不去游泳。

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

基於這些理由，風險評估的基準是很棘手的。

and reporting on risk can be misleading,

風險報告可能會誤導，

especially when it shares some numbers in absolute terms

尤其，有些數值是以絕對風險表示，

and others in relative terms.

而有些是以相對風險表示。

Understanding how these measures work

了解這些測量標準

will help you cut through some of the confusion

可以幫助你克服困擾，

and better evaluate risk.

並且更好地評估風險。