In today’s world of conflicting interests, flawed science, and sensationalized media, it’s important to question new claims and findings, especially when those findings could have serious implications for your health. One of the most important things you can do to make sure you’re getting the real scoop (aside from reading my articles, of course!) is to read the scientific literature yourself.
I know many of my readers are already adept at perusing research with a critical eye, but for those of you delving into the recesses of PubMed for the first time, it can be a little overwhelming. In this article, I’ll cover the basics of epidemiological research, practical tips for navigating the scientific literature, and things to watch out for when reading studies.
Types of Scientific Research
Let’s start by going over the different types of studies and papers you’ll find in scientific journals. At the broadest level, we can categorize research into either review papers or original research. These can also be classified as secondary or primary research, respectively.
Review papers are similar to the articles you’ll find here on this blog. They draw on prior research that has been published on a specific topic, and use that research to form their own broader conclusions. Reviews will usually give you some background on a topic, which is helpful, and they’re generally much easier to read and understand than original research.
Original research comprises anything that is based on the researcher’s original findings, whether through experiments in a lab or population-wide data collection. This graphic illustrates some types of studies you’ll encounter.
Experimental studies are the only type of study that can potentially establish causation, because the researchers actually manipulate variables instead of just observing a natural sequence of events. Randomized controlled trials are a type of experimental study, and are often called the ‘gold standard’ of scientific research.
Observational studies are the other main type of original scientific research, and they’re often criticized for inferring causation from correlation when they have no business doing so. I’ll now describe some of the main types of observational studies, using cancer as an example.
Cohort studies look at a group of people with similar characteristics (a ‘cohort’) who are free from the outcome being studied (cancer, in this case). The people in the cohort are measured for different risk factors at baseline, and then followed over a period of time to see which people develop cancer. Then researchers can see if having a certain risk factor at baseline increased people’s chances of developing cancer in the future.
Cohort studies can be further broken down into prospective and retrospective studies, based on when the study was conducted in relation to when the data was collected. The study described above would be a prospective study. A retrospective cohort study would use data from the past to define a cohort and measure baseline risk factors, and then look at present-day data to see which people in the cohort already developed cancer.
A case-control study on cancer would divide participants into two groups based on whether they have cancer (the ‘cases’) or not (the ‘controls’). Researchers would then look back in time using existing data to see if they could pinpoint any factors that could potentially explain the different outcomes.
Cross-sectional studies observe a population at a single moment in time, measuring both the occurrence of the disease in question and various risk factors. For example, researchers might look to see if populations who live closer to polluted urban areas have a higher rates of cancer.
Navigating the scientific literature
Now that you know what to expect, the next step is actually finding the papers you want to read! I probably don’t have to tell you that if you’re looking for peer-reviewed articles, a Google or Yahoo search just won’t cut it. However, Google does have a search engine called Google Scholar that is helpful for finding studies and papers. It pulls results from online journals and databases around the web, and is pretty good about only providing papers from trustworthy sources. PubMed is also a great place to look for trustworthy articles.
You might run into difficulty accessing the full text for papers you find. Sometimes, Google Scholar or PubMed will link to free full-text versions, but often you’ll be asked to pay for the full text, which can be pricy. A good way to access the full text is though a university library, because they usually have subscriptions to online journals and academic databases. If you’re a college or graduate student or otherwise have access to a university library, take advantage of the wealth of resources at your disposal! Some medical libraries on college campuses are also accessible to the general public.
After you find one study or paper dealing with your topic of interest, I’d recommend looking at the ‘related citations’ section to find other relevant articles. The ‘cited by’ section is also quite helpful, because it shows articles that cite the paper you’re looking at. That way, you can see how other researchers have interpreted it, and how the study is being used to support various claims. These features are especially valuable if you’re having trouble finding the results you want using keywords.
When researching a topic, it’s usually a good idea to begin with review articles. The authors have already done the hard work of rounding up relevant studies and drawing conclusions from them, and reviews are a great starting point for looking deeper into the original research. If you don’t want to rely on the authors to draw conclusions for you, however, you can look at the reference lists and read the individual studies for yourself!
Reading the scientific literature
Here are some questions to ask and things to watch out for when reading original research:
- Was the study short-term or long-term? Many interventions are only effective in the short run, and have completely different effects over the long term. Fish oil is a good example of this.
- Was the study done in vitro or in vivo? In humans or animals? Clearly, an in vitro study (one done in a petri dish or test tube) isn’t necessarily generalizable to living beings, and studies done in animals don’t necessarily apply to humans.
- Was the methodology strong? If a trial isn’t randomized, double blind, or placebo-controlled, its results might not be as dependable. In observational studies, it’s vital to ensure that different variables are controlled for. You should also watch out for general cluelessness and dishonesty.
- Did they use surrogate markers or end points? Surrogate markers, such as cholesterol numbers and other lab test values, are often used to measure the effectiveness of a treatment. However, changes in surrogate markers don’t necessarily translate to changes in overall disease outcomes. If a study uses values from a blood or urine test to measure success rather than using disease progression or mortality, it’s good to be wary.
- Absolute risk or relative risk? Sometimes, scientists like to use relative risk to make their results sound more impressive. If a treatment reduces the risk of a disease from 2% to 1%, the absolute risk reduction is 1%. Treatment or no treatment, your absolute risk of getting the disease is pretty small. However, you could also truthfully say that the treatment reduces the risk by 50%. This sounds more impressive, but it gives a skewed impression of how valuable the treatment actually is.
Here are some things to watch out for when reading review articles:
- Do the citations support the claim? When a paper cites sources for a claim it makes, scroll down to the reference list and look up some of the references. You might be surprised by how often citations are misused. The results may have been taken out of context, or generalized to populations where the results don’t apply. Occasionally, researchers will completely misinterpret a study!
- Do the authors infer causation from correlation? Make sure that when a review paper claims that ‘x’ causes ‘y,’ they’re not citing an observational study. (Some good examples of this can be found in my special report on red meat.)
- How do the conclusions compare with other reviews? Different review articles on the same topic can come to very different conclusions. Often, their reference lists will look remarkably similar! It’s sobering how frequently researchers can look at the same evidence but come to completely different conclusions. In these situations, you might have to delve deeper into the original research.
- Who funded the review? It’s always a good idea to see who funded a study or a review, and what the authors’ affiliations are. If a paper comes off as biased to begin with, a questionable source of funding could further tarnish its credibility.
Those are some of my best tips for being a critical consumer of the scientific literature. Reading research for yourself can be both empowering and sobering, and you’ll likely discover that most things aren’t as clear-cut as we may want them to be. Just be aware that there is such a thing as information overload, and be careful not to fall so far down the research “rabbit hole” that you stress yourself out!
Do you read peer-reviewed scientific publications? What do you look for in a well-written study or review? Share your thoughts in the comments!