Cohort study

A cohort study follows patients who received different treatments, or have different characteristics, and compares what happened to them. A prospective cohort is followed forward from the start; a retrospective cohort is assembled from records after the fact. Either way, the patients or their dentists chose the treatment, rather than a random assignment.

That is the weakness. Patients who received one treatment may differ from those who received another in ways that also affect the outcome, so a cohort study can show an association but has a harder time proving cause than a randomized controlled trial. Its strengths are length of follow-up and real-world practice, which is why so much of what is known about how long restorations last comes from cohorts.

Ryan's research articles say when a question rests on cohort evidence alone. In the recall-interval article, for example, the periodontal-maintenance literature is cohort studies only, with no randomized trials, so its conclusions carry less weight than the trial evidence for healthy adults.