But ignoring that difference for a minute, consumption is just the demand at the "equilibrium" point. Nobody is counting all the times someone goes and says "gee, I want to buy some more of this but it's too expensive" or "gee, I wanted to buy some more of this but the shelves were empty" and so the demand curve is unobserved (unobservable?). Similarly with the supply curve.
AFAIK...
The only market where anything like textbook supply and demand applies is a Dutch auction. Er, well, nearly
Dutch auction - Wikipedia, the free encyclopedia
This type of auction is convenient when it is important to auction goods quickly, since a sale never requires more than one bid. Theoretically, the bidding strategy and results of this auction are equivalent to those in a Sealed first-price auction; however, experiment indicates that a Dutch auction typically results in lower sale prices [1].
See perfect competition.
Theoretically, the 'lead time' it takes for a producer to change supply to deal with shifting demand is not relevant on a market when there is a sufficiently large number of producers and consumers (there also is no real shifting demand on a market with perfect competition).
When people talk of a "shift" in demand or supply they normally refer to a movement of the whole curve. So half the time "demand" refers to the whole "demand curve" and the rest of the time it refers to a quantity (apparently, to "consumption"). Similarly Supply" can refer to "supply curve" or to "production". We have met the enemy, and he is us — Pogo
The point is, of the model only the intersection of the curves, that is, the clearing price and the production = supply = demand = consumption are observable. We have met the enemy, and he is us — Pogo
The idea that economic theory is necessarily about prediction, like celestial mechanics applied to planets, is the source of much confusion. "Economists can't predict recessions, so what do they know?" The answer is that, in principle, they could know a lot, yet never predict a price or an economic fluctuation.
That economists commonly think they know many things that aren't actually true is a different issue. Words and ideas I offer here may be used freely and without attribution.
