The eight levels of scientific evidence on the infographic are here, in order from weakest to strongest:
- Anecdotes: Stories about personal experiences and perceptions are notoriously unreliable and should not be considered scientific evidence.
- Expert opinions: Experts often base opinions on evidence, but they also sometimes offer informed perspectives on issues about which the data isn’t yet clear. Remember: experts aren’t immune to bias.
- Case reports / case series: These are studies on one person (case report) or a group of people (series) with similar clinical characteristics. These are common with new diseases or health conditions and are used as a first step to gather more information.
- Cross-sectional studies: Observational studies in this category capture the health of a particular group of people at a single point in time. These provide useful snapshots, but they can’t show what caused a disease or ailment.
- Case-control study: These observational studies compare groups of people with a specific condition with other groups of people who do not have that condition. It’s important to remember, however, that some correlations in these groups could be the result of coincidence.
- Cohort study: These large, long-term observational studies look at what causes diseases in different groups (cohorts) by following a group that shares a specific factor or exposure to a risk factor over time to see if they develop a mutual ailment.
- Randomized trial: This is the gold standard for testing health claims. Participants are randomly divided into control groups (no treatment) and test groups (receive a given treatment) to minimize the influence of bias.
- Meta-analysis and systematic review: These are not studies themselves, but rather analyses and reviews of a group of studies, filtered for quality and rigor. In other words, these reviews collect studies about a subject or question, then narrow