This article does a good job explaining what went wrong with randomization in the recently retracted Mediterranean diet study and how it affects the results.

New England Journal Of Medicine Retracts And Replaces Mediterranean Diet Study : Shots - Health News : NPR https://www.npr.org/sections/health-shots/2018/06/13/619619302/errors-trigger-retraction-of-study-on-mediterranean-diets-heart-benefits