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** Investment is affected by the output and the [[interest rate]] (i). Consequently, we can write it as I(Y,i). Investment has positive relationship with the output and negative relationship with the interest rate. For example, when Y goes up, the investment will increase.
* despesa del govern ('''G''').
* gross [[government]] investment and consumption expenditures ('''G''').
* [[Netexportacions worth|net]] [[exports]]netes ('''NX''' and sometimes= ('''X-M''')), i.e.,demanda netexterior demandde byproducció the rest of the world for the country'sdel outputpaís.
In sum, for a single country at a given time, aggregate demand ('''D''' or '''AD''') = '''C''' + '''I<sub>p</sub>''' + '''G''' + '''(X-M)'''.
These [[macrovariables]] are constructed from varying types of [[microvariables]] from the price of each, so these variables are denominated in (real or nominal) [[currency]] terms.
Provided to you by Matthew Martin
==Two Concepts of the "Aggregate Demand Curve" ==
Understanding of the aggregate demand curve depends on whether it is examined based on changes in demand as income changes, or as price change.
===Keynesian Cross===
{{main|Keynesian cross}}
[[Fitxer:KeynesianCross11 2007.png|thumb|250px|Keynesian cross diagram]]
In the "Keynesian cross diagram," a ''desired total spending'' (or ''aggregate expenditure'', or "aggregate demand") curve (shown in blue) is drawn as a rising line since consumers will have a larger demand with a rise in disposable income, which increases with total national output. This increase is due to the positive relationship between consumption and consumers' disposable income in the [[consumption function]]. Aggregate demand may also rise due to increases in investment (due to the [[accelerator effect]]), while this rise is reduced if imports and tax revenues rise with income. Equilibrium in this diagram occurs where total demand, '''AD''', equals the total amount of national output, '''Y''', (which corresponds to total national income or production). Here, total demand equals total supply.
In the diagram, the equilibrium level of output and demand is determined where this desired spending curve intersects a line that represents the equality of total income and output ('''AD=Y'''). The intersection gives the equilibrium output, '''Y''''.
The movement toward equilibrium is mostly via changes in inventories inducing changes in production and income. If current output exceeds the equilibrium, inventories accumulate, encouraging businesses to cut back on production, moving the economy toward equilibrium. Similarly, if the level of production is below the equilibrium, then inventories run down, encouraging an increase in production and thus a move toward equilibrium. This equilibration process occurs when the equilibrium is stable, i.e., when the '''AD''' line is less steep than the '''AD=Y''' line.
The equilibrium level of output determines the equilibrium level of employment in the model. (In a dynamic view, these are connected by [[Okun's Law]].) There is no reason within the model why the equilibrium level of employment should correspond to [[full employment]]. Bringing in other considerations may imply this correspondence, though.
If any of the components of aggregate demand ('''C''' + '''I<sub>p</sub>''' + '''G''' + '''NX''') rises at each level of income, for example because business becomes more optimistic about future profitability, that shifts the entire '''AD''' line ''upward''. This raises equilibrium income and output. Similarly, if the elements of '''AD''' fall, that shifts the line downward and lowers equilibrium output. (The '''AD=Y''' line does not shift under the definition used here).
===Aggregate Demand-Aggregate Supply model===
Sometimes, especially in textbooks, "aggregate demand" refers to an entire demand curve that ''looks'' like that in a typical [[Marshallian demand|Marshallian]] [[supply and demand]] diagram.
Thus, that we could refer to an "aggregate quantity demanded" ('''Y<sup>d</sup>''' = '''C''' + '''I<sub>p</sub>''' + '''G''' + '''NX''' in real or inflation-corrected terms) at any given aggregate average price level (such as the [[GDP deflator]]), '''P'''.
In these diagrams, typically the '''Y<sup>d</sup>''' rises as the average price level ('''P''') falls, as with the '''AD''' line in the diagram. The main theoretical reason for this is that if the nominal [[money]] supply ('''M<sup>s</s>''') is constant, a falling '''P''' implies that the [[Real vs. nominal in economics|real]] money supply ('''M<sup>s</s>'''/'''P''')rises, encouraging lower [[interest rate]]s and higher spending. This is often called the "[[Keynes effect]]."
Carefully using ideas from the [[theory]] of [[supply and demand]], [[aggregate supply]] can help determine the extent to which increases in aggregate demand lead to increases in real [[output (economics)|output]] or instead to increases in prices ([[inflation]]). In the diagram, an increase in any of the components of '''AD''' (at any given '''P''') shifts the '''AD''' curve to the right. This increases both the level of real production ('''Y''') and the average price level ('''P''').
But different levels of economic activity imply different mixtures of output and price increases. As shown, with very low levels of [[real vs. nominal in economics|real]] [[gross domestic product]] and thus large amounts of unemployed resources, most economists of the [[Keynesian economics|Keynesian school]] suggest that most of the change would be in the form of output and employment increases. As the economy gets close to [[potential output]] ('''Y*'''), we would see more and more price increases rather than output increases as '''AD''' increases.
Beyond '''Y*''', this gets more intense, so that price increases dominate. Worse, output levels greater than '''Y*''' cannot be sustained for long. The '''AS''' is a ''short-term'' relationship here. If the economy persists in operating above potential, the '''AS''' curve will shift to the left, making the increases in real output transitory.
At low levels of '''Y''', the world is more complicated. First, most modern industrial economies experience few if any falls in prices. So the '''AS''' curve is unlikely to shift down or to the right. Second, when they do suffer price cuts (as in Japan), it can lead to disastrous [[deflation (economics)|deflation]].
===Marxian critique===
In [[Marxian economics]], the equation of aggregate demand with expenditure on GDP or GNP is rejected as false, on conceptual and statistical grounds.
Firstly, GDP as a measure of [[value added]] excludes purchases of all [[intermediate consumption|intermediate goods]] used up in production. Even so, gross value added cannot be simply equated with final demand, insofar as it excludes transfers and most trade in second-hand items.
Secondly, [[Gross Output]] from which GDP is derived by deducting intermediate expenditures, encompasses only those flows of income or expenditure regarded as ''related to production''. ''Property income'' in the form of certain types of interest, transfers, land rents and realised capital gains from asset sales are excluded from gross output and GDP. Therefore, if the amount of property income (or transfers) increases, although GDP remains constant, national income receipts can nevertheless increase, and consequently aggregate demand can also increase.
Thirdly, [[Gross fixed capital formation]] measures only investment in productive fixed assets and does not constitute total investment, which includes also purchases of financial assets.
Fourthly, GDP in principle excludes sales of second-hand assets except for those modified by some prior productive activity (e.g. reconditioned cars).
Finally, expenditure on GDP obviously disregards the creation of credit money by banks and governments, which boosts aggregate demand.
Thus, it is argued, the catch-all Keynesian notion of aggregate demand:
*obscures the distribution of income between [[social classes]] with different propensities to save, consume and invest, and
*fails to differentiate appropriately between different kinds of investment and consumption expenditure.
Restraining consumption and a higher savings rate does not automatically imply more investment, and lower investment does not automatically mean higher consumption expenditure. Funds may (as Keynes himself acknowledges) be hoarded.
==See also==
* [[Aggregation problem]]
* [[Aggregate supply]]
* [[Reproduction (economics)]]
* [[List of economics topics]]
==External links==
* Elmer G. Wiens: [http://www.egwald.ca/macroeconomics/keynesian.php Classical & Keynesian AD-AS Model] - An on-line, interactive model of the Canadian Economy.
*[[Greg Mesaros]]: [http://www.ewinwin.com/corptest/eWinWin-WhitePaper2.pdf How US Manufacturers are Using Demand Aggregation to Increase Sales & Lower Costs] - Real-time demand aggregation in a B2B application over the Internet.
== References ==
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{{esborrany d'economia}}
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