In medicine, there are few certainties when it comes to cause and effect.
Diseases can take a long time to develop and people tend overall to live a long time.
And during their lives they're exposed to an almost endless series of experiences that might alter their risk of developing a particular condition (diet, social class, lifestyle, economics, education and so on).
So, the only way we can be reasonably certain about the risks is to observe vast numbers of people over a long period of time and record what happens to them.
It's costly, it's unbelievably slow but it works and it's the bedrock of medicine.
Without the numerous evidence-based discoveries that have come out of decades of longitudinal science, medicine would be stuck in the dark ages.
In this programme, Dr Ben Goldacre, medic and author of Bad Science explores the past, present and future of longitudinal research.
How did these monumental long term-studies come about?
What have we learned from them and what