Table 4 Summary of the distinction proposed by Dekkers et al. [28]

Cohort study: Patients are sampled on the basis of exposure. The occurrence of outcomes is assessed during a specified follow-up period.

Case series: Patients with a particular disease or disease-related outcome are sampled. Case series exist in 2 types:

1. Sampling is based on a specific outcome and presence of a specific exposure.

2. Selection is based only on a specific outcome, and data are collected on previous exposures. Cases are reported regardless of whether they have specific exposures. This type of case series can be seen as the case group from a case–control study .