The research, based on the US-wide National Education Longitudinal Survey, found that a teacher's gender had "large effects on student test performance".

BBC: NEWS | UK | Education | Male teachers 'better for boys'

So Dr. Mark Daniel DeBoer, an associate professor of pediatric endocrinology at the University of Virginia School of Medicine, and his colleagues turned to a large database of 10, 700 children in the Early Childhood Longitudinal Survey, Birth Cohort to investigate the relationship between the type of milk children drank and their body mass index (BMI).

CNN: Drinking skim milk may not lower child obesity risk

Here, a study called the Cebu Longitudinal Health and Nutrition Survey has been going on since 1983.

ECONOMIST: Vaccination

Over time, The CMO Survey will develop a longitudinal database and provide more definitive answers to the questions surrounding marketing excellence.

FORBES: In Search of Marketing Excellence: Ten Differences Between High-Performing and Low-Performing Companies