But, with so many studies indicating a relationship between coffee and decreased the risk of death, it seems very likely that the effect exists.
Additionally, it is challenging to research any connection with longevity in humans within an experimental setting. In this case, doing so would involve getting people to make changes to their coffee drinking habits and then following them for many years.
Not only would that be expensive and impractical, but it might also be unethical.
So, despite their limitations, observational studies are the main tool we have for looking at this connection – and they do strongly suggest that coffee can decrease the risk of death.