Cohort Study
Definition
A study design that follows prospectively over time one or more populations (called cohorts) to determine which patient characteristics (risk factors) are associated with the development of a disease or outcome. As the study is conducted, the outcome from participants in each cohort is measured and relationships with specific characteristics determined.
Advantages
- Subjects in cohorts can be matched, which limits the influence of confounding variables
- Standardization of criteria/outcome is possible
- Easier and cheaper than a randomized controlled trial (RCT)
Disadvantages
- Cohorts can be difficult to identify due to confounding variables
- No randomization, which means that imbalances in patient characteristics could exist
- Blinding/masking is difficult
- Outcome of interest could take time to occur
Design pitfalls to look out for
The cohorts need to be chosen from separate, but similar, populations.
How many differences are there between the control cohort and the experiment cohort? Will those differences cloud the study outcomes?
Fictitious Example
A cohort study was designed to assess the impact of sun exposure on skin damage in beach volleyball players. During a weekend tournament, players from one team wore waterproof, SPF 35 sunscreen, while players from the other team did not wear any sunscreen. At the end of the volleyball tournament players' skin from both teams was analyzed for texture, sun damage, and burns. Comparisons of skin damage were then made based on the use of sunscreen. The analysis showed a significant difference between the cohorts in terms of the skin damage.
Real-life Example
This study uses data collected from high school students from Baltimore, Maryland, and studies the differences in initiation of tobacco use between a cohort of adolescents that started working for pay and a cohort of adolescents that did not work. The results suggest that adolescents who work for pay have a higher risk of initiating tobacco use.
To study effects of hospitalists, general internists, and family physicians on patient care, patients that were hospitalized with certain conditions under the care of hospitalists, general internists, and family physicians were separated into three cohorts. The results showed that patients cared for by hospitalists had shorter hospital stays and lower costs than those cared for by general internists or family physicians.
To determine the long-term effectiveness of influenza vaccines in elderly people, cohorts of vaccinated elderly and unvaccinated community-dwelling elderly were studied. The results suggest that the elderly who are vaccinated have a reduced risk of hospitalization for pneumonia or influenza.