Studies have shown that taking vitamins is good for your health and bad for your health.
研究表明，吃维生素有利于健康，也有害于健康。
That newly discovered herb can improve your memory or destroy your liver.
最新发现的一种药草可以增强记忆力，或者损害你的肝脏。
Headlines proclaim a promising new cancer treatment and never mention it again.
头条新闻报道了一种前景光明的癌症治疗方法，接着却再也没有提及。
On a daily basis, we are bombarded with attention-grabbing news,
每天我们被吸睛的新闻所轰炸，
backed up by scientific studies, but what are these studies? How are they performed?
它们都有科学研究的支持，但这些研究到底是什么？它们是如何进行的？
And how do we know whether they're reliable?
我们怎么知道它们是否可靠？
When it comes to dietary or medical information,
当涉及饮食和医疗信息时，
the first thing to remember is that while studies on animals or individual cells can point the way towards further research,
首先需要记住的是，虽然实施在动物或单个细胞上的研究可以引导未来的研究，
the only way to know how something will affect humans is through a study involving human subjects.
但想知道它们如何影响人类的唯一方法是通过一个有人体参与的研究。
And when it comes to human studies, the scientific gold standard is the randomized clinical trial, or RCT.
当涉及人体研究的时候，科学的黄金标准就是随机临床试验，或者叫RCT。
The key to RCTs is that the subjects are randomly assigned to their study groups.
RCT的关键是被试者被随机分配到各个研究组。
They are often blinded to make them more rigorous.
他们通常是不知情的，这样可以使研究更加严谨。
This process attempts to ensure that the only difference between the groups is the one the researchers are attempting to study.
这个过程是为了保证不同组之间的唯一区别是研究者想要研究的东西。
For example, when testing a new headache medication,
举个例子，当测试一种全新的治疗头痛的药物时，
a large pool of people with headaches would be randomly divided into two groups,
一大群有头痛问题的人会被随机分配到两组，
one receiving the medication and another receiving a placebo.
一组得到治疗的药物，另一组得到安慰剂。
With proper randomization, the only significant overall difference between the two groups
在正确的随机分配下，两组间唯一显著的区别
will be whether or not they received the medication, rather than other differences that could affect results.
就是他们有没有得到药物，而不是其他会影响结果的因素。
Randomized clinical trials are incredible tools,
随机临床试验是非常有用的工具。
and, in fact, the US Food and Drug Administration often requires at least two to be conducted before a new drug can be marketed.
事实上，美国食品及药物管理局通常要求在新药上市前要进行至少两次随机临床试验。
But the problem is that an RCT is not possible in many cases,
但是问题是在很多案例里随机临床试验是不可能的，
either because it's not practical or would require too many volunteers.
这可能是因为它不切实际，或者是因为需要过多的志愿者。
In such cases, scientists use an epidemiological study, which simply observes people going about their usual behavior,
在这类情况下，科学家们使用一种流行病学研究，它简单地观察人们的日常行为，
rather than randomly assigning active participants to control invariable groups.
而不是通过随机分配主动参与者来控制不变量。
Let's say we wanted to study whether an herbal ingredient on the market causes nausea.
假设我们想研究一种市场上的草药成分是否会引起恶心。
Rather than deliberately giving people something that might make them nauseated,
我们不会故意给人们一些可能造成他们恶心的东西，
we would find those who already take the ingredient in their everyday lives. This group is called the cohort.
而是找到那些在日常生活中已经服用了这种成分的人。这个组叫做队列。
We would also need a comparison group of people who do not have exposure to the ingredient.
我们还需要一个比较组，他们并没有接触过这种成分。
And we would then compare statistics.
接着我们就要比较数据。

If the rate of nausea is higher in the herbal cohort, it suggests an association between the herbal supplement and nausea.
如果恶心程度在草药队列里面偏高，它就表明了在草药成分和恶心之间有一种联系。
Epidemiological studies are great tools to study the health effects of almost anything,
流行病学研究非常有用，可用于研究几乎任何东西对健康的影响，
without directly interfering in people's lives or assigning them to potentially dangerous exposures.
而不需要直接干扰人们的生活，也不会让人们接触到有潜在危险的东西。
So, why can't we rely on these studies to establish causal relationships between substances and their effects on health?
那么，我们为什么不依赖这些研究来研究物质和它们对健康的影响之间的因果关系？
The problem is that even the best conducted epidemiological studies have inherent flaws.
问题是即使是实施得最好的流行病学研究也存在内在的缺陷。
Precisely because the test subjects are not randomly assigned to their groups.
准确的说是因为被试者不是被随机分配到他们的组别的。
For example, if the cohort in our herbal study consisted of people who took the supplement for health reasons,
比如在我们的草药研究中，如果队列是由那些因为健康原因而服用草药成分的人所组成的，
they may have already had higher rates of nausea than the other people in the sample.
他们可能本身就比在另一个组里的人更有可能感到恶心。
Or the cohort group could've been composed of people who shop at health food stores
或者说队列组有可能由那些在健康食品商店购物的人组成，
and have different diets or better access to healthcare.
或者由不同饮食习惯的人组成，或者由那些享有更好的医疗保健的人组成。
These factors that can affect results, in addition to the factor being studied, are known as confounding variables.
这些在被研究因素之外也可能影响到结果的因素，被称为混肴变量。
These two major pitfalls, combined with more general dangers, such as conflicts of interest or selective use of data,
这两个主要的缺陷加上更多的常规性问题--比如利益冲突或是选择性地使用数据，
can make the findings of any particular epidemiological study suspect,
能让任何流行病学的发现变得可疑，
and a good study must go out of its way to prove that its authors have taken steps to eliminate these types of errors.
而一个好的研究必须不厌其烦地来证明它的研究者采取了必要的步骤来消除各种类型的错误。
But even when this has been done, the very nature of epidemiological studies,
但是，即使这些都做到了，流行病学研究的本质
which examine differences between preexisting groups, rather than deliberately inducing changes within the same individuals,
是研究已经存在的不同组别的差异，而不是在相同群体内特意加入差别，
means that a single study can only demonstrate a correlation between a substance and a health outcome,
这意味着一个单独的研究只能证明一种物质和一个健康结果之间的一种关联，
rather than a true cause and effect relationship.
而不是一个真的因果联系。
At the end of the day, epidemiological studies have served as excellent guides to public health,
最后我想说，流行病学研究在公共健康中起到了非常大的指导作用，
alerting us to critical health hazards, such as smoking, asbestos, lead, and many more.
警告我们某些严重的健康威胁，比如吸烟、石棉、铅还有更多。
But these were demonstrated through multiple, well-conducted epidemiological studies, all pointing in the same direction.
但是这些都是通过多个实施良好的流行病学研究来证明的，而所有这些研究都指向同一结果。
So, the next time you see a headline about a new miracle cure
所以，下次你看到一个关于某种全新神奇的治疗的头条新闻，
or the terrible danger posed by an everyday substance, try to learn more about the original study
或是关于某种日常用品产生的可怕威胁，试着去看一下原始的研究，
and the limitations inherent in any epidemiological study or clinical trial before jumping to conclusions.
去了解流行病学研究和临床试验中内在的局限性，而不是直接跳过去读结论。