Consider the case where a natural disaster or some other event disrupts production at a particular facility. When things return to normal, is there a period of catch-up, where production is temporarily higher than normal? Or does production just revert to the previous normal level, in which case the production that would otherwise have occurred in the disrupted period is simply lost?
Or is the effect on production levels more permanent, so that the growth rate returns to normal, but the level of production now follows a persistently lower trend track? These lumps and bumps can affect how you interpret the data, and they're not always about natural disasters. A recent example can be seen in public sector wages growth.