The difficulty is compounded because the reasons for the behavior that leads to heightened potential impact of a low-probability event often are not fully understood - for example, why the markets cracked or why the economy has not responded more strongly to previous stimulus.
Moreover, when the threat of the low-probability event recedes into the past, policy may need to compensate over time in the other direction to preserve economic and price stability - to achieve the appropriate policy stance on average - a consideration that further complicates the policy decision in these circumstances.