Epidemiology deals with the occurrence, distribution, and control of disease in a population. Epidemiological research looks at populations to investigate potential links between areas such as diet, lifestyle, genetics, or other factors within populations, and diseases. Cancer epidemiological research looks at how many people have cancer; who gets specific types of cancer; and what factors (such as environment, job hazards, family patterns, and personal habits, such as smoking and diet) play a part in the development of cancer.

Epidemiological research often compares two groups of people who are alike except for one factor, such as breast cancer to try to determine if any factor is associated with the disease. The findings of epidemiological research do not generally relate to individuals but instead are intended to provide information on broad population groups.

Why should we do epidemiological research?

Traditionally, epidemiology has been a guiding force behind many public health policies and programs worldwide. The decline in infectious diseases and the rise in the relative importance of non-communicable diseases (diseases that are not contagious and that cannot be transmitted from person to person) have led to the development of modern epidemiology. The last five decades have witnessed enormous progress in the fight against cancer, much of which stems from direct contributions from epidemiological research.

Since the development of frameworks for population risk assessment in the 1970s and 1980s, cancer epidemiologists have provided much of the evidence for modern cancer prevention strategies. These include the control of tobacco smoking and public and professional education concerning alcohol drinking, diet, occupational and environmental exposures, and avoidance of cancer associated infections. Epidemiologists have also played a key role in the design and conduct of intervention trials and prospective studies of screening tools to detect cancer precursors and early cancers. To a large extent, the contributions of epidemiologists have also raised the standard for judging the evidence in favour or against specific cancer screening and preventive interventions.

What types of research questions answered by epidemiological research?

Epidemiologists work on issues from the practical, such as outbreak investigation, environmental exposure, patterns of care, cancer site specific risk factors and health promotion, to the theoretical, including the development of statistical, mathematical, philosophical, and biological theory. To this end, epidemiologists employ a range of study designs to answer questions such as:

  • What are the causes of cancer?  
  • Is screening effective in reducing the risk of dying from cancer?  
  • What are the trends in cancer?  
  • Which persons are at greatest risk?  
  • What places have higher cancer rates?

Limitations of epidemiological research

The most frequent flaws in epidemiology are bias and confounding. Bias refers to something that may lead a researcher to wrong conclusions; for example, mistakes or problems in how the study is planned, or how the information is gathered or looked at. Bias produces incorrect results and can rarely be corrected after data collection. Confounding refers to the inability to tell between the separate impacts of two or more factors on a single outcome. For example, one may find it difficult to tell between the separate impacts of genetics and environmental factors on depression. Confounding can be addressed during statistical analysis (number crunching), provided that sufficient information was gathered during the research study.

Cost

Epidemiological studies tend to be larger and have a longer time frame than other scientific experiments. They also often involve the active cooperation of patients. Patient recruitment and assessment take time and money. As does the cost of collection and testing of biological samples if required, telephone calls, postage, and personnel resources. Furthermore, if mistakes are detected during the course of the research, repeating it would be expensive and time consuming.

Who conducts epidemiological research?

Epidemiologists work in a variety of settings including universities, research institutions, hospitals and government organisations. Some epidemiologists work in the field, such as in the community, commonly in a public health service, and are at the forefront of investigating and combatting disease outbreaks.

Epidemiologic research methods

There are various types of research approaches within epidemiological research including prospective studies that monitor a group of people over a period of time to observe the effects of diet, behaviour, and other factors on health or the incidence of cancer and retrospective studies that look at events and behaviours that have already taken place.

Epidemiological research may also be either observational or experimental. In experimental research, part of the population receives some sort of treatment (sometimes called an intervention) and the results are compared with the results for those who do not receive the treatment (the control group.) Observational research suggests associations or correlations between characteristics based on observed differences. To determine cause and effect, experimental research must be conducted.

Epidemiological research designs are summarised below. Each design is ranked according to what was traditionally thought of as its usefulness in providing accurate results. This ranking is sometimes referred to as the hierarchy of evidence, with stronger evidence coming from the studies later rather than earlier on in the list:

  • Case series - what clinicians see, this is simply a description of cases.  
  • Ecological - geographical comparisons, compares groups of people not individuals; assumes that associations seen on a group level also hold on an individual level  
  • Cross sectional - survey, a snapshot in time 
  • Case-control - compare people with and without a disease (retrospective)  
  • Cohort - follow people over time to see who gets the disease (prospective) 
  • Randomised controlled trial (RCT) - This is a human experiment, where people are randomly assigned to receive one treatment or the other. Often called the gold standard of epidemiological studies

Reproduced with the kind permission of Cancer Council New South Wales.