epidemiology

It is oft-repeated that correlation does not imply causation. But it does. That's precisely why epidemiologists and economists are so fascinated by correlations. Thus, it is far more accurate to say that correlation does not prove causation.

There are two major reasons for this. The first is because of confounders, hidden factors that are the true causes of the observed effect. For instance, one might be tempted to conclude that moving to Florida makes people develop Alzheimer's. But this correlation has been confounded by age; in reality, old people both retire to Florida and develop Alzheimer's. The Sunshine State is blameless....

Isn't it odd that Florida has so many people living with Alzheimer's Disease? If Erin Brockovich was investigating the case, she probably would conclude that it's something in the water.

The rest of us, however, know that there's nothing especially dangerous about Florida.* Old people all across America move to Florida for retirement, and Alzheimer's is a disease associated with aging. Ergo, Florida is not causing Alzheimer's; aging is.

Using the vernacular of epidemiology, aging in this example is known as a confounding variable. The apparent association between living in Florida and having Alzheimer's is confounded by age. If a researcher did not take age into consideration, he would draw incorrect conclusions about a link between geography and Alzheimer's....

Smoking really is as bad as everybody says it is.

A person's chance of getting lung cancer depends on how many years one has smoked, as well as how many cigarettes one has smoked per day. In general, according to the International Journal of Cancer, smoking makes a man nearly 24 times more likely to get lung cancer and a woman almost 8 times more likely. Put another way, smoking increases a man's risk of lung cancer by 2,300% and a woman's by 700%.

Lung cancer isn't the only thing a smoker needs to worry about. Smoking is linked to several different cancers, and it damages the cardiovascular, respiratory, and immune systems. Indeed, as the...

Microbiologist Stanley Falkow is credited with saying, "The goal of every bacterium is to become bacteria." Similarly, the goal of every pathogen is to infect a new host.

Pathogenic microbes face an evolutionary trade-off: On the one hand, they want to be as nasty as possible, because the nastier they are, often the easier it is to spread from one host to the next. Think of Vibrio cholerae, the causative agent of cholera, which upon infection can result in a person producing gallons of diarrhea. Death is due to dehydration, but not before the patient served as an incubator and excreted billions of more bacteria into the environment.

On the other hand, a microbe does not want to be too nasty. If it incapacitates or kills its host quickly, it will be difficult for...

When Erin Brockovich, an environmental activist, shook down Pacific Gas & Electric for $333 million for allegedly poisoning a community with hexavalent chromium and causing cancer and all sorts of other health problems, Julia Roberts portrayed the protagonist in a sensationalized blockbuster movie. It is unlikely, however, that Hollywood will be filming a sequel.

Why? Because not only was Ms. Brockovich wrong, but the State of California has now partially repudiated what she fought for.

Erin Brockovich, Junk Scientist

We've known for a long time that Ms. Brockovich used junk science to score a jackpot settlement. She used a common rhetorical trick, known as the Texas...

A new study in the Journal of the American Medical Association found that 99% of donated brains from people who played in the NFL showed signs of chronic traumatic encephalopathy (CTE), a type of neurodegenerative disease that is thought to result from persistent brain injury from knocks to the head. Signs of CTE were also noted in the brains of 91% of people who played the sport in college and 21% who played in high school.

Predictably, headline writers went crazy:

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Nutrition science is notoriously unreliable. The reason is because a substantial proportion of research in the field is conducted using surveys, and people just aren't very good at remembering what and how much they ate. 

The field is further damaged by a sensationalist press, which breathlessly reports every study and converts minor findings into flashy, eye-catching headlines. The latest example of this is a study that linked increased coffee consumption to reduced mortality. In general, media outlets wrote headlines like, "Drinking coffee may reduce the risk of death."

Well, not exactly. A plethora of data shows that coffee probably has some health benefits. However, after reading the original paper, carefully examining the data, and applying a dose of common sense (...

The media is alive with a new report sure to satisfy the confirmation bias of a billion people; drinking coffee is good for you!! And evidently, the more, the better.

At least that is what the media has chosen to tell us in their interpretation of an article in the journal Annals of Internal Medicine entitled Association of Coffee Consumption With Total and Cause-Specific Mortality Among Nonwhite Populations.

Aren't these the same media groups that decades ago lifted Center for Science...

When I was learning how to do word problems in elementary school, we were taught to ask ourselves a few questions after arriving at an answer. One of them was, "Does my answer make sense?" For example, if a problem asked how many apples a person can buy if he has $5 and each apple costs 50 cents and your answer is negative 17 million, something has gone horribly wrong.

Does my answer make sense? is such a great question, that every published scientific paper henceforth should be required to answer it explicitly. Perhaps we would avoid seeing papers with titles like this:

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Every student in America should be required to take a class called, "What Do We Know, and How Do We Know It?" Perhaps if we learned from an early age how we know the things we claim to know, fewer Americans would fall for ridiculous conspiracy theories.

Public health is a field that is widely misunderstood, even by science journalists. That is because epidemiology is an inexact science that is complicated by a large variability in the quality of the data it produces, as well as by its reliance on advanced statistical methods. Let's leave the latter aside and focus on the former. Which epidemiological studies are most reliable and why?

From weakest to strongest, here are the most common epidemiological study designs:

Case report. A case report is...