They can't insulate an economy from an external shock or bad policy choices by other actors.

The state has a vital role in helping the economy to weather an immense external shock.

This has improved recently and tends to lead the economy by six months, suggesting stronger economic performance in late 2012 and early 2013 — barring an external shock.

It was the right policy to help employees and businesses to withstand a huge external shock and contain the damage to the economy's long-term productive capacity.

Yet, after a while, forests can become so densely connected that they become increasingly vulnerable to an external shock, such as fire, which can spread very fast.