The key ratio warning of trouble ahead for the US

The US is still the world's largest economy. If it sneezes, the rest of the world gets a cold. So how can we best predict what's about to happen in America? There are plenty of economic indicators around. But watching them all can end up giving confusing signals. So it's a good idea to stick to the tried and tested measures. One such measure has worked well for years. What's more, as technology becomes an increasingly large part of our lives, its efficiency as a leading indicator is only improving. And right now, it's blaring out distress signals – which is bad news for investors on this side of the Atlantic too.