In 1951, Richard Doll and Austin Bradford-Hill initiated a groundbreaking research endeavor by contacting all registered doctors in the UK to inquire about their smoking habits. The British Doctors Study enrolled and tracked more than 40,000 participants, monitoring mortality rates and causes of death in the subsequent years and decades. Even with the release of the initial set of preliminary findings in 1954, there was already evidence linking smoking to lung cancer and increased mortality. As time went on, the study yielded further conclusive evidence regarding the health risks associated with smoking. Additionally, it expanded its scope to investigate other causes of death (such as heart disease) and explore different behavioral variables (like alcohol consumption).
The Doctors Health Survey is renowned as one of the largest, most ambitious cohort studies, illustrating the value of this approach in enhancing our understanding of disease risks. However, cohort studies possess a broader range of applications as a research method. This article offers an overview of cohort studies, highlighting the opportunities and challenges they present to researchers, as well as their role in advancing the evidence base for nursing and healthcare more broadly.
What are cohort studies?
Cohort studies are a type of longitudinal study that involves tracking research participants over an extended period, often spanning many years. These studies specifically recruit and follow individuals who share a common characteristic, such as a particular occupation or demographic similarity. Throughout the follow-up period, some members of the cohort will be exposed to a specific risk factor or characteristic. By measuring outcomes over time, researchers can investigate the impact of this variable, such as establishing the association between smoking and lung cancer in the British Doctors Study. Cohort studies are particularly valuable in epidemiology, as they contribute to our understanding of factors that increase or decrease the likelihood of developing diseases.
The British Doctors Study is just one example of numerous large-scale, long-term cohort studies conducted to enhance our knowledge of disease causes and inform evidence-based guidelines for healthier living. For instance, the Framingham Heart Study, which began in 1948 and now includes the third generation of participants, has provided extensive data on cardiovascular disease risk factors and has influenced international prevention guidelines. Similarly, the Nurses’ Health Study, now in its third generation and involving approximately 280,000 participants across the United States, has offered significant insights into health risks such as the association between obesity and certain cancers, as well as the link between shift work and an increased likelihood of developing chronic diseases.
While large epidemiological research studies often receive the most attention in cohort studies, this method has other applications beyond that. In nursing research, cohort studies have examined the progression of nurses throughout their education and careers. For example, Li and his colleague as part of the European NEXT study group, recruited nearly 6,500 female nurses who initially expressed no intention to leave the profession. The study followed this cohort for a year and identified that 8% developed the intention to leave nursing, often due to factors like inadequate salary or limited opportunities for promotion.
Typically, cohort studies should adhere to an observational approach. However, some studies labeled as cohort studies explore the effectiveness of specific interventions. For instance, Lansperger and his colleague investigated nurse practitioner (NP)-led critical care in a large university hospital in the USA. They collected data on all patients admitted to the intensive care unit over a three-year period. Patients from this cohort received care from teams led by either doctors or NPs, and outcomes, primarily 90-day mortality, were monitored. By comparing the two groups, the researchers concluded that outcomes were similar regardless of whether patient care was led by a doctor or an NP.
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Strengths and weaknesses of cohort studies
Cohort studies are a robust method for establishing causal relationships, as they allow researchers to draw confident conclusions regarding the association between risk factors and diseases. These studies are often conducted on large-scale populations, enabling the identification of timelines in which specific behaviors contribute to the development of diseases. Additionally, since participants are usually free of the disease at the start of the study, cohort studies are particularly useful in understanding the progression of behaviors and their impact on disease over time.
However, conducting cohort studies presents challenges. Collecting prospective data from thousands of participants over many years or decades is complex, time-consuming, and expensive. Participants may drop out, introducing potential bias, and their behavior may be influenced by the awareness of being part of a study cohort. Analyzing data from these large-scale studies is also challenging due to the presence of numerous confounding variables, making it difficult to establish causality. Furthermore, when cohort studies are linked to specific interventions, such as in the case of Lansperger and his colleague study on nursing practitioner-led critical care, the lack of randomization in assigning participants to different arms of the study reduces the robustness compared to randomized controlled trials.
One approach to reduce the time required for cohort studies is to conduct them retrospectively. This pragmatic approach allows for quicker completion by utilizing historical data. For example, Wray and his colleague used a retrospective cohort study to identify factors associated with non-continuation of students in nursing programs. By examining characteristics in five previous student cohorts, they discovered that factors such as older age and local residency were linked to higher levels of continuation.
However, retrospective cohort studies carry the risk of sampling bias, as there is a higher likelihood of missing data. The available data fields are not specifically designed for the study, requiring researchers to make use of whatever data are available, potentially compromising the study’s quality.
Reporting and critiquing of cohort studies
When reporting a cohort study, it is highly recommended to adhere to the STROBE (STrengthening the Reporting of OBservational studies in Epidemiology) guidance. STROBE is an international collaborative initiative that involves experts with extensive experience in organizing and disseminating observational studies, including cohort studies. Its objective is to improve the quality of reporting in epidemiological research.
Critical appraisal is crucial when evaluating any cohort study to identify its strengths and weaknesses, as well as to assess the usefulness and validity of the study findings. Key components of critical appraisal for cohort studies include evaluating the study design in relation to the research question, assessing the methodology employed, evaluating the appropriateness of the statistical methods used, considering potential conflicts of interest, and determining the relevance of the research findings to clinical practice.
Cohort studies play a fundamental role in epidemiological research, providing valuable insights into disease risk factors through the examination of large populations over extended periods. The evidence derived from these studies forms the basis for disease prevention guidelines followed by healthcare professionals worldwide. Well-known cohort studies like the British Doctors Study, the Framingham Heart Study, and the Nurses’ Health Study have had a significant impact on shaping healthcare practices.
Beyond epidemiology, cohort studies offer opportunities for research in various fields, including nursing. In nursing research, this approach is particularly useful in investigating factors that influence students’ progression through their educational programs or nurses’ career trajectories.
While cohort studies present certain challenges, such as their size, complexity, and long duration, these can be overcome with careful planning and implementation. Despite the obstacles, cohort studies have the potential to make substantial contributions to the development of evidence-based healthcare.