A new drug reduces the risk of heart attacks by 40%. Shark attacks are up by a factor of two. Drinking a liter of soda per day doubles your chance of developing cancer.
「新藥降低心臟病發風險四成」 「鯊魚襲擊事件翻倍」 「日飲一升汽水,罹癌機會倍增」
These are all examples of relative risk, a common way risk is presented in news articles. Risk evaluation is a complicated tangle of statistical thinking and personal preference. One common stumbling block is the difference between relative risks like these and what are called absolute risks.
以上都是相對風險的例子 也是新聞內容中慣用的風險陳述方法 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 剛才說的「相對風險」與「絕對風險」 兩者的區別在哪裡,難倒不少人
Risk is the likelihood that an event will occur. It can be expressed as either a percentage— for example, that heart attacks occur in 11% of men between the ages of 60 and 79— or as a rate— that one in two million divers along Australia’s western coast will suffer a fatal shark bite each year. These numbers express the absolute risk of heart attacks and shark attacks in these groups. Changes in risk can be expressed in relative or absolute terms. For example, a review in 2009 found that mammography screenings reduced the number of breast cancer deaths from five women in one thousand to four. The absolute risk reduction was about .1%. But the relative risk reduction from 5 cases of cancer mortality to four is 20%. Based on reports of this higher number, people overestimated the impact of screening.
「風險」是將來會發生某件事的可能性 表達的方式可以是百分比 例如:11% 的 60 至 79 歲男性 會心臟病發作 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 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 風險變化也可用相對或絕對數字表示 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 從每一千名女性中 5 宗降至 4 宗 絕對風險下降了約 0.1% 但癌症死亡個案由 5 宗減至 4 宗 相對風險就下降了 20% 引用了這個較高的數字 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, let’s consider the hypothetical example of a drug that reduces heart attack risk by 40%. Imagine that out of a group of 1,000 people who didn’t take the new drug, 10 would have heart attacks. The absolute risk is 10 out of 1,000, or 1%. If a similar group of 1,000 people did take the drug, the number of heart attacks would be six. In other words, the drug could prevent four out of ten heart attacks— a relative risk reduction of 40%. Meanwhile, the absolute risk only dropped from 1% to 0.6%— but the 40% relative risk decrease sounds a lot more significant.
為了理解兩種表達風險的方法 所造成的差別有多大 可以看看一個假想範例: 一款新藥減少心臟病發的 風險為 40% 在一個沒吃新藥的 1,000 人群體中 10 人會心臟病發 絕對風險是千分之十或 1 % 假設有相似的 1,000 人吃了新藥 有 6 人心臟病發 換言之,服用新藥能避免 十分之四的人心臟病發作 相對風險下降 40% 此時,絕對風險僅從 1% 降至 0.6% 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, or any other negative outcome, is worthwhile— isn’t it? Not necessarily. The problem is that choices that reduce some risks can put you in the path of others. Suppose the heart-attack drug caused cancer in one half of 1% of patients. In our group of 1,000 people, four heart attacks would be prevented by taking the drug, but there would be five new cases of cancer. The relative reduction in heart attack risk sounds substantial and the absolute risk of cancer sounds small, but they work out to about the same number of cases.
心臟病發等劫數,一宗都嫌多 能避就避,總值得一試吧? 也不一定 問題是選擇降低某些風險時 可能會讓你暴露在其他風險之下 假設新藥會導致 1% 的 病人中的半數罹癌 在我們那 1,000 人群體中 吃新藥能避免 4 宗心臟病發 卻新增 5 宗罹癌個案 心臟病發的相對風險看似下降很多 癌症的絕對風險也看似很小 但屈指一算,實際數字其實差不多
In real life, everyone’s individual evaluation of risk will vary depending on their personal circumstances. If you know you have a family history of heart disease you might be more strongly motivated to take a medication that would lower your heart-attack risk, even knowing it provided only a small reduction in absolute risk. Sometimes, we have to decide between exposing ourselves to risks that aren’t directly comparable. If, for example, the heart attack drug carried a higher risk of a debilitating, but not life-threatening, side effect like migraines rather than cancer, our evaluation of whether that risk is worth taking might change. And sometimes there isn’t necessarily a correct choice: some might say even a minuscule risk of shark attack is worth avoiding, because all you’d miss out on is an ocean swim, while others wouldn’t even consider skipping a swim to avoid an objectively tiny risk of shark attack. For all these reasons, risk evaluation is tricky at baseline, and reporting on risk can be misleading, especially when it shares some numbers in absolute terms and others in relative terms. Understanding how these measures work will help you cut through some of the confusion and better evaluate risk.
在現實生活中 每個人的自我風險評估各異 取決於個人的狀況 如果你知道家族有心臟病史 你也許會有較強的動機去服藥 減少自己心臟病發的風險 就算明知絕對風險只下降一點點 有時候,我們要在各種 無法直接比較的風險中做取捨 譬如,治心臟病的新藥 讓身體衰弱的機率提高,卻不會致命 有偏頭痛的副作用,但不會致癌 這些都會影響到 我們考量承擔何種風險 有時並無所謂的正確抉擇: 即使鯊魚襲擊的風險微乎極微 但有人能免則免 並不稀罕錯失一場海洋暢泳 但有人卻不會為了那微不足道的 鯊魚襲擊風險,而放棄游泳 因為上述理由,風險評估其實很棘手 風險報告亦可能誤導民眾 尤其是當絕對數字與相對數字 混雜呈現時 了解風險評估如何運作後 能夠協助你釐清疑惑 將風險評估做得更好