Observational epidemiology studies are often the source of highly misleading results. And yet, despite this problem, they are essential to the better understanding of human health. There are many exposures, some of them quite critical, that cannot be studied in any other way.
My personal favorite is adverse drug effects. Clinical trials are often underpowered to detect adverse events; in order to show these effects trials often need to be combined. Given the logistics involved, it is helpful to show an association between the drug in question and adverse events in real populations.
I hope to discuss the many interesting challenges and ideas in this field of research as I try to muddle towards some sort of resolution in a confusing sea of fuzzy data.