What researchers mean by... epidemiology

The cornerstone of public health, epidemiology investigates which groups in a population are affected by disease, and why

If you’ve ever wondered whether vegetarians live longer than meat-eaters, or why some people suffer from chronic pain and others don’t, or what the health consequences are of working nights, you’re asking the same questions asked by epidemiologists—researchers who work in the field of epidemiology.

Epidemiology is considered the basic science of public health. In simple terms, it’s the study of who gets sick and why. “Epidemiology” literally means “the study of what is upon the people.” The word comes from the Greek epi, meaning “upon,” demos, meaning “people,” and logos, meaning “study.

In the early days, epidemiology concentrated on studying diseases such as cholera. Today, epidemiology is applied to all kinds of health-related conditions—diseases (e.g. influenza, cancer, depression), health problems (e.g. obesity, high blood pressure), injuries (e.g. work-related, traffic-related) and social problems (e.g. gambling, domestic violence). Its role is to describe who is affected by these conditions, why, and what can be done to treat and prevent them.

Population versus individual

A distinguishing feature of epidemiology is that it studies health-related conditions at the population level, as opposed to the individual level. A good way to understand this is to compare the differing approaches of clinicians and epidemiologists to diseases.

Doctors and other clinicians are largely concerned with the effects of disease within a single person. They work one-on-one with patients to diagnose problems and determine what can be done to make them healthier.

Epidemiologists, on the other hand, are concerned with how diseases affect society as a whole. They study groups of people to diagnose and respond to illnesses in populations: how many are affected (i.e. prevalence), who is affected and why (i.e. determinants of health), and what works and what doesn’t to cure or prevent these illnesses at a societal level (e.g. treatment protocols, public health interventions).

Let’s look more closely at how epidemiologists carry out their studies of disease and other conditions. To understand the “who,” epidemiologists seek to describe what part of the population is affected. How does the prevalence of a disease vary by age, sex, ethnicity, income, geography, work role and so on? This analysis goes well beyond demographics.  It might relate to genetic disposition, childhood exposure, living conditions and more.

Difficult to find cause

Understanding who gets sick is often the first step in learning what factors might be behind why people get sick. Sometimes, epidemiologists rely on other fields of science to get to the “why.” They might learn from geneticists that certain types of people are predisposed to an illness. That might then lead them to probe more deeply about other factors that might protect certain individuals within that group from the disease.

Although epidemiologists seek to understand the why, they rarely get to say “because.” Researchers must clear many hurdles before they can pronounce the cause of a health outcome. How strong is the association between event A and outcome B? Does A always occur before B? Does B always follow A? If A is altered in some way, is B altered too, and to the same degree? The more researchers can say yes to these questions, the closer they get to being able to claim A is the cause of B.

These criteria for causation should give you an idea why epidemiological studies are so difficult to carry out. They’re also why epidemiologists are often so circumspect when stating the findings of their research.

Many of the terms associated with epidemiological studies are covered in other “What Researchers Mean By ...” columns. For example, epidemiological studies can be observational or experimental, retrospective or prospective. Experimental studies include randomized controlled trials; observational studies include cross-sectional studies, cohort studies and case-control studies. For more on these terms, go to: www.iwh.on.ca/what-researchers-mean-by.

Epidemiological studies are important. They form the bedrock for sound public health policies and strategies, thus protecting and improving the health of entire populations.

Source: At Work, Issue 73, Summer 2013: Institute for Work & Health, Toronto