I agree with Nick, although I would say it's not an exact science. Plenty of scientific evaluation is far from 100% certain, and this is something non scientists struggle with. Man made climate change, for example, is scientific but only around 85% certain (we'd be nuts to go for the other 15% but plenty do!). I think a lot of the media are to blame for teaching us that the world is full of black & white certainties. As with climate change, there are many variables & many unintended consequences of action or inaction & many people on both sides of the camp have a vested interest in telling us what is best.