Why are there limitations to calculating national GDP (national income accounting)?

All movements in the business cycle are measuredby the
rate of growth ofthe real gross domestic product (GDP). A nation’s nominal GDP measures
its economic output; the real GDP is the nominal GDP adjusted for inflation. Three
limitations to calculating GDP are: The first limit centers on random, external shocks
to the economic system. These so-called “exogenous shocks” include both negative,
recession-inducing events as well as positive, expansionary-enhancing “productivity
shocks.” The second limit is typically viewed as inherently stable. Yet the value can be
off base due to policy errors and miscalculations or, in the worst case, by
Machiavellian politicians using the powers of incumbency to enhance their re-election
fortunes. The third limit relies on much more complex and systemic view of the economy.
It is characterized by the “co-movements” of many variables.