Talk:Demand curve/Archives/2014

First comments

The reference to a "downward slope" does not match the graph. Usually, an independent variable is shown horizontally (x-axis). The image in the article's top-right corner is poorly labeled and incomprehensible to all but specialists.

I teach math, I it took me quite a long time to figure out the graph. Let's make it easier for the layman, eh? --Uncle Ed 11:13, 16 July 2007 (UTC)

When describing demand & supply curves in economics we put price on the y-axis and quantity on the x-axis. For demand curves, price is the independant variable; for supply curves, quantity is the independant variable. The status quo is to put price on the left - the graphs would look much the same either way. I do, however, take umbrage at the fact that the plots on the graph are not straight lines. the labelling could do with some improvement too.--poorsod^talk 18:42, 6 January 2008 (UTC)

The aggregation of individual demand curves to derive a market demand curve is dependent on the assumption that each consumer is an idependent, idiosyncratic decision maker. This assumption means that the consumptiond decisions of one consumer do not affect the consumption decisions of any other consumer; a wholly unrealistic assumption. If you drop this assumption in the interest or realism it is no longer possible to aggregate individual demand functions. To illustrate you ask A how many widgets he would buy at a certain price and A responds 15. You then ask B how many widgets she would buy at the same price and B says 20. Upon learning of B's preferences A not to be outdone by B says I have changed my mind and would buy 21 widget. These feedback loops make aggregation impossible. Traditional economic theory also assumes that the consumption preferences, tastes, of consumers are fixed - in other words no firm would advertise. —Preceding unsigned comment added by 70.220.140.142 (talk) 12:53, 8 April 2009 (UTC)

Hello poorsod,

Are you sure that quantity is the independent variable on a demand curve? If it was, then producers would be deciding how to price their products based on the quantity in the market, correct? In that case, then if the quantity of supply goes up, then the producers would decrease their prices in anticipation of heavy competition, right? But that's just the opposite of what the supply curve shows! The curve shows the number of items produced increasing as the price goes up! I think maybe the function goes the other way.

Let's say that the current price for carrots is too high. Unfortunately, nobody knows this yet. The suppliers look at the prices and say "Look, we can make a killing in carrots!" So they produce a lot.

It's only afterward that the suppliers vote with their pricing to help change the market price to something lower.

Let's say that the current price for carrots is too low. Unfortunately, nobody knows this yet. The suppliers say "Carrots are no good right now, let's grow green beans instead." So they produce a little.

Again, it's only afterward that the suppliers vote with their pricing to help change the market price to something higher.

The suppliers don't know the equilibrium amount. Instead, they use the current market price as a proxy for the purposes of figuring out how much to produce. Then they adjust their prices to reflect their experience. Lots of price adjustments add up, moving the market price closer to the equilibrium price. If the market price is exactly at the equilibrium price, then using the current market price as a proxy works very well and the price system is doing its job.

Does that make sense? I'm only just learning this stuff. Tigerthink (talk) 03:38, 13 October 2008 (UTC)

Hi everybody,

In truth, economists both use models where price is the dependent variable and where price is the independent variable. The decision to put price on the vertical axis is just due to the convention which was established many years ago. I admit that in the years since I started studying economics I have been confused many times as a result of this convention.

On a slightly different topic, the curved supply and demand functions are perfectly acceptable. In earlier years of study they are often drawn as straight lines for simplicity but it would be incorrect to assume that they are always straight lines. (K Doerr (talk) 01:03, 5 December 2008 (UTC))

increasing marginal utility

Anomalous examples probably don't belong in the lead. I'm not sure where they really do belong, since this article is so brief now. Also, it's probably important to note that the weird implications for the individual demand curve probably don't carry over to the (more important in most cases) market demand curve. CRETOG8(t/c) 14:19, 2 September 2008 (UTC)

I added the section Characteristics.--Patrick (talk) 14:40, 2 September 2008 (UTC)

Thanks. That should work for now, although boy, this article needs a lot of work. CRETOG8(t/c) 14:48, 2 September 2008 (UTC)

Tentative Correction

OK, I am just learning this stuff, but this bit didn't seem quite right: "equilibrium price(the price at which all sellers are able to find a willing buyer, also known as market clearing price)". Shouldn't it be the maximum price at which all sellers are able to find a willing buyer? For example, let's say we're at equilibrium and the sellers decide to be generous and decrease their prices. They'll sell all their inventory, but it won't be equilibrium price, will it? Tigerthink (talk) 03:08, 13 October 2008 (UTC)

I fixed it.--Patrick (talk) 12:04, 13 October 2008 (UTC)

Aggregate Demand

the introductory paragraph includes the following sentence, "The demand curve for all consumers together follows from the demand curve of every individual consumer: the individual demands at each price are added together." There are many problems with aggregating individual demand curves to generate a market demand curve. First, you can aggregate individual demand curves only if you assume that preferences are fixed or if you assume that consumers are independent idiosyncractic decision makers. in other words, a consumer is not influenced by the cosumption choices of others. his assumption hardly comports with reality. If the assumption does not hold then aggregation is impossible because a consumer's decisions are dependent on the decisions of others. Example, A is asked how many pairs of shoes he would buy at a certain price. A says at that price i would be willing and able to buy 2 pairs of shoes. B is asked the same question and says 4 pairs. Questioner goes back to A and says B is willing to buy four pairs of shoes, what do you think about that? A says if B has any interest in those shoes then i have none. Or A, not to be outdone by B says then i'll buy five pairs. And on and on. --Jgard5000 (talk) 10:36, 18 September 2009 (UTC)jgard5000

A more diificult problem is the that it is impossible to predict the effect of a change in relative prices on social utility. At the societal level a change in relative prices involves a redistribution of real income and one cannot state what the net effect of this redistribution will be as far as the demand for the good in question. As Varain noted, "The aggregate demand function will in general possess no interesting properties..." Keen, Debunking Economics (Zed 2004) at 40. While Keen's well placed attacks on neoclassical theory seemed to have been ignored by orthodox economist, Varian is highly regarded.

Finally - it would seem that if individual consumers have different preferences, as they surely must, then individual demand curves would be oriented differently and in aggregating demand you would have situations in which a person's demand for a paricular good was so relatively small or unusual that it would not count for purposes of aggregation.--Jgard5000 (talk) 14:35, 19 September 2009 (UTC)jgard5000 Or it may be that your aggregate demand curve would be kinked. Or maybe this is not an issue at all.--Jgard5000 (talk) 14:39, 19 September 2009 (UTC)jgard5000.

Trying to hit on your various points. First aggregate demand is (unfortunately for terminology) something different than aggregated demand. I'm sure this is distinction is missed by well-educated and well-meaning people, but it makes me wary of evaluating Varian's statement as being about micro demand.

Second, to get the market demand, you add up individual demand-that idea has the benefit of being both simple, and generally true. You can add things without those things being independent of each other, so if Jim always wants to eat what Mary's eating, you just have to take that into account when adding.

Your more difficult problem is true, but that's a matter of general equilibrium versus partial equilibrium analysis, it's a decision one has to make when doing the analysis. For most things, we think it's reasonable to stick to partial equilibrium. For some issues like labor, we worry that we might need to go to general equilibriumm and your critique comes into play explicitly.

And your final points don't seem like they would impact aggregation in any general way. If someone's demand is negligible, then it will be a negligible part of the market demand. CRETOG8(t/c) 21:19, 19 September 2009 (UTC)

I am trying to find the Varian text to check the context of the quotation. I did find a book by David Kreps, A Course in Microeconomic Theory (Princeton 1990). On page 62 & 63 in a section titled “Aggregate Demand” Kreps states,

“...total demand will shift about as a function of how individual incomes are distributed even holding total (societal) income fixed. So it makes no sense to speak of aggregate demand as a function of price and societal income.

…

So what can we say about aggregate demand based on the hypothesis that individuals are preference/utility maximizers? Unless we are able to make strong assumptions about the distribution of preferences or income throughout the economy (everyone has the same homothetic preferences for example) there is little we can say. ..”

This sounds very much like Varian’s statement but I haven’t the expertise to properly evaluate it. So I would welcome your thoughts. In the mean time I hope to find the Varian text. I am sure that Varian has extensive writings in macro theory but I do believe he is best known for his work in intermediate and advanced micro theory. --Jgard5000 (talk) 06:46, 22 September 2009 (UTC)jgard5000

As to independence requirement - it is possible that the feedback loops will be negative and the effect will eventually peter out but it is also possible that positive feedbacks will develop in which case in would be quite difficult to add things up. The necessity of independence is implicitly addressed in the assumption of the standard model that all economic actors are independent, idiosyncratic decision makers and the assumption that preferences and tastes are fixed at least in the short run,--Jgard5000 (talk) 11:53, 22 September 2009 (UTC)jgard5000

A lot going on here. I belatedly realized that the Varian quote Keen gives is from a book I have on my shelf, so I've found it myself. It does refer to (what I call) market demand rather than the macro aggregate demand, so you're correct in referring to it. I've asked for help at the Econ WikiProject on how to best disambiguate "aggregate demand". I'll think about the details of the arguments separately. CRETOG8(t/c) 03:19, 23 September 2009 (UTC)

Here's what i think is Keen's argument - whether Varian and Kreps would agree is far beyond me

The law of diminishing marginal utility is the foundational principle of the neoclassical theory of consumer behavior. Briefly succinctly stated: each person has a utility function U = f(x) that they are utility maximizers that is there singular objective is to Maximize utility - achieve the highest level of satisfaction given his income (budget constraint). Given these assumptions as Keen concedes “the economic model of how an individual [maximizes utility] is intellectually watertight.” 24

The problem is in aggregating utility to achieve a social utility function. When a person decides to buy a banana in doing so he is effectively redistributing income from someone else to him the effects on overall satisfaction utility are impossible to predict. Altering distribution of income does alter social welfare - two strong assumptions necessary for the contrary assumption to he true - that everyone has the same tastes and that each person’s taste remain the same as income changes so each additional income is spent exactly the same way as all previous dollars.

As Keen notes these first assumption amounts to assuming that there is a single consumer the second that there is a single good. The implications for traditional economics - you cannt draw conclusions anout social welfare - you cannot derive a market or aggregate consumer demand function - Adam Smith was wrong.--Jgard5000 (talk) 12:59, 23 September 2009 (UTC)jgard5000

What does this mean for traditional ecoomic world? - Fire and brimstone coming down from the skies! Rivers and seas boiling; Forty years of darkness! Earthquakes, volcanoes The dead rising from the grave Human sacrifice, dogs and cats living together... mass hysteria! The return of the Brady Bunch Variety Hour!!! --Jgard5000 (talk) 13:04, 23 September 2009 (UTC)jgard5000

OK, here's roughly where I am on all this now: There's no problem at all in having a market (aggregate) demand curve, and getting it by aggregating individual demand curves, IF we take that at face value. There are problems with using the market demand curve as if it were an individual demand curve. Some of the problems don't apply to a single-commodity demand curve, anyway. Some (like what do you do about how wealth enters the function) aren't vital in many applications because you can consider wealth variable just as tastes are variable. The main potential risk is in welfare analysis, and I'm back to banging my head on that a bit.

In any case, where I come out on this is: (a) yes, you can aggregate demand curves, but (b) you have to be somewhat careful in using market demand analogously to individual demand. (b) is likely to not be relevant for this article in its current state, since most of the cautions are about deeper theory than this page aims to cover. CRETOG8(t/c) 05:32, 25 September 2009 (UTC)

I will agree on one point the problems of aggregation of individual demand curves to derive a market demand curve are too advanced for this article. I do believe that there are major problems but we can agree to disagree and the whole point of this is to make the article better. --75.202.240.236 (talk) 18:09, 25 September 2009 (UTC)jgard5000 One final point - you simply cannot ignore the fact that a change in relative prices changes the distribution of aggregate income - it is possible to derive an aggregate demand curve but in order to have one that is smoth negatively sloped well behaved and so on requires you to make the "strong" asumptions mentioned by Varian and others or you can relax those assumptions in which case you get a demand curve that is generally downward sloping but which twists and tuns, is kinked, has flats spots and regions of rising demand. This is what Varian was referring to when he talked about "no restrictions on aggregate behavior in general." Keen Debunking Economics at 44 quoting Varian.

One fimal final point. Ifwe assume the derivation of aggregare consumer demand curves want they necessarily be convex to the origin rather than linear - and if so woulldn't the MR curve also be convex?--Jgard5000 (talk) 23:45, 25 September 2009 (UTC)jgard5000

Why wouldn't our discussion be a proper subject for a separate paper i get the filling sometimes that the power that be would prefer that this topic not be discussed.--Jgard5000 (talk) 23:49, 25 September 2009 (UTC)jgard5000

When you move to an aggregate consumer demand curve three additional parameters can affect the shape and position of the curve - (1) the number of consumers (2) the distribution of tastes among the consumers (3) the distribution of income among consumers with differenct tastes. Binger & Hoffman, Microeconomics with Calculus, 2nd ed. (Addison-Wesley 1998) at 154. If relative prices change then there is a concommitant change income and unless persons have the same tastes a redistribution of income which means that the aggregate or market function could have all sorts of "weird" shapes.--75.203.45.142 (talk) 18:04, 1 October 2009 (UTC)jgard5000

Characteristics

"The demand curve usually slopes downwards from left to right" While this is true for an individual demand curve it is not true for an aggregate demand curve. An aggregate demand curve may have a positive, negative or zero slope.--Jgard5000 (talk) 12:13, 18 September 2009 (UTC)jgard5000

I'm not sure if you mean aggregate demand or market demand. If you mean market demand, then various things can happen (including demand being a correspondence rather than a function), but we believe the curve 'usually slopes downwards. CRETOG8(t/c) 11:43, 21 September 2009 (UTC)

I mean market demand in the sense of the horizonatl summation of individual demand curves. I don't mean aggregate demand in the macro sense.--Jgard5000 (talk) 06:55, 22 September 2009 (UTC)jgard5000 again i do not think this need be discussed at this level and i don't want to engage in improper discussions.

Shift in Demand

The section begins with the following statement, "The shift of a demand curve takes place when there is a change in the relationship between quantity and price that is brought about by a change in any of the factors influencing demand except price. A demand shift results in a new demand curve[3]." Shift in a demand curve are caused by a change in a non-price determinant of demand. The shift is due to a change in the x intercept because the non-price determinants are "curled up" in the constant term of the demand equation.--Jgard5000 (talk) 14:49, 19 September 2009 (UTC)jgard5000 It should be noted that according to convention economists graph the demand equation with price on the y axis and quantity on the x axis - in other words backwards. the equation that is actually graphed is the price or inverse demand equation.--Jgard5000 (talk) 20:30, 19 September 2009 (UTC)jgard5000. After reading the sentence again the author is correct. The call on the field is reversed. A shift in the demand curve is caused by a change in a non price determinant of demand, The shifted curve is a new demand curve.--Jgard5000 (talk) 20:27, 25 September 2009 (UTC)jgard5000

constant term

Recent edits have added things like, "The other determinants of demand are being held constant which means they are bundled up in the constant term." A demand function doesn't strictly need to have a constant term at all, and if it does, that doesn't mean the constant term contains all information about non-price determinants of demand. One could have the demand function $Q_{1}=a+(p_{1}-p_{2})^{2}p_{1}$. In this case, when plotting the demand function, we'd hold $p_{2}$ constant, yes, but $p_{2}$ is not in the constant term. CRETOG8(t/c) 11:39, 21 September 2009 (UTC)

There has to be a mathematical explanation for a shift in the demand curve and a shift triggered by a change in a NPDD is undoubtedly due to a change in the constant term beacuse there is no other parameter or variable that could account for it. As i implied I am not an ecnomist and possibly my explanation is limited to linear eqauations.--Jgard5000 (talk) 07:04, 22 September 2009 (UTC)jgard5000 Also i may be using the phrase "constant term" too broadly' I do not meand all factors that are being held constant but the intercept.--Jgard5000 (talk) 07:06, 22 September 2009 (UTC)jgard5000 One final thought the NPDD are the other thing that are being held constant the cereis paribus assumption. It is of course impossible to specify all factors that influence individual consumption decisions. Also look at section on manipulating demand curves.--Jgard5000 (talk) 07:18, 22 September 2009 (UTC)jgard5000

I knew I had read an explanation why the demand curve shift is a NPDD changes. I found my old managerial econ textbook = the Samuelson and Marks text fourth edition page 83 - 86.

“…[I]t is important to remember that in the background all other factors affecting demand are held constant….But what happens if there is a change in one of the other factors affecting demand? …]S]uch a change causes a shift in the demand curve.”

Samuelson and Marks then use an example to illustrate what happens. Begin with a demand function Q = f(P, Po, Y) The full demand equation is Q = 25 + 3Y + P0 - 2P where Po and P = $240 and income in the area equals 105. If we put these values into the equation we get Q = 580 - 2P the standard demand curve. Then assume income increases to 119. Put the new values into the equation, the old equation becomes the new demand equation is Q = 622 - 2P

Comparing the new equation with the old the only difference is in the x intercept it has increased by 42 due to a change in income a NPDD. As the authors state, [the equations] are the same with one key difference. The constant term of the new demand equation is larger than that of the old.”

On page 85 they illustrate this change in demand with a graph that clearly shows that the increase in demand due to an increase in income is due to a change in the constant term. Contrary to what I said before this would apply to any demand equation involving more than one variable. As Samuelson also notes you can graph a demand curve using a two dimensional graph but you cannot graph a demand function of four variables - the fourth axis would have to be at right angles to the other three axes. With a two dimensional graph and an equation in four variables the other three variables have to be somewhere and where they are is the constant term - I apologize for the reference to bounded or curled up - too much string theory.--Jgard5000 (talk) 09:03, 22 September 2009 (UTC)jgard5000

OK, we may only be disagreeing on precise phrasing, rather than substance. Just need to phrase it so that it's clear that demand shifts come from changes in "ceteris paribus" things, but not be specific about the structure of those shifts. I'll try to check back here in a few days and if you haven't done it by then, I'll give it a shot. CRETOG8(t/c) 10:21, 22 September 2009 (UTC)

I really think that using the Samuelson example or the less elegant one that appears in the article gives the reader a valuable insight into why shifts occur and helps to make the distinction changes in demand and changes in quantity demanded. --Jgard5000 (talk) 18:22, 22 September 2009 (UTC)jgard5000

When a non-price determinant of demand changes the curve shifts. These "other variables" are part of the demand function. They are "merely lumped into intercept term of a simple linear demand function." [6] the quoted language is from the Samuelson Marks instructor's manual--Jgard5000 (talk) 11:57, 8 July 2010 (UTC)jgard5000--Jgard5000 (talk) 11:57, 8 July 2010 (UTC)

parameters

Change in demand" often refers to change of the curve, due to a change in one of these factors other than the price of the commodity itself. It is e.g. a shift, or a change of slope. I don't think that a change in an NPDD would change the slope since the assumption is that the variables are independent. A change in a NPDD would be reflected in a shift in the demand curve - in or out. A change is the slope could be caused bu a change in the units of measurement or a change in the a determinant of PED. Of course i didn't go to Princetom.

Of course, it doesn't matter where you went. In any case, I think that in most examples of shifts in demand are like you say, but those are only for examples. It's like examples tend to have linear demand and supply functions, even though we expect linearity would be very unusual-it makes for easy examples which make the point. I'm not familiar with the assumption that different determinants of demand are independent, particularly if that independence is supposed to mean they're additive in the demand function. I really think this stuff should be left out. One of the reasons supply & demand are often expressed with pictures instead of math is the pictures can be nonparametric-we can get the points across without tying ourselves down to some specific, limited set of possible demand functions. CRETOG8(t/c) 02:46, 22 September 2009 (UTC)

upward-sloping demand

At Supply and demand, I removed a section on upward-sloping demand (diff). The section read as very WP:OR, but it could possibly be salvaged. If so, it should still be in an article (like this one) primarily on demand rather than supply & demand. CRETOG8(t/c) 04:07, 23 September 2009 (UTC)

need to add

Having pulled a bunch of stuff out, stuff which needs to be added:

• pictures illustrating shifts
• basic info on individual vs market demand curves
• line to aggregate demand
• stuff on elasticity CRETOG8(t/c) 07:04, 25 September 2009 (UTC)

I agree. i think there should be a section on double shift, i agree that discussion of aggregate consumer demand curve should be limited to principles or intermediate level discurrion rather tahn ecposing these folks to the dirty little secrets - i also agree on elastitcity which is the most useful concept encountered in intro courses. By the way how do you do pictures - this is still new to me i know google has tons of graphs but what c-right restrictions apply. --Jgard5000 (talk) 23:40, 25 September 2009 (UTC)jgard5000

Does horizontal summation across individual demand curves depend on the individual curves having the same slopes. If you attempted to sum demand curves that had varying slope - in effect superimposed one over the other you may well an almost certainly would have an aggregate curve with kinks. Now as the number of curves being aggregated increased the kink would smooth out and the demand curve would appear to be convex to the origin rather than linear. --Jgard5000 (talk) 19:22, 27 September 2009 (UTC)jgard5000speaking of aggregate demand curves shouldn't the firm demand curves also add up to the market demand curve?--Jgard5000 (talk) 19:22, 27 September 2009 (UTC)jgard5000

Demand Curve

Why there is no typical demand curve for oligopolistic firm?

Assumptions

If economics wants to pretend to be a science, shouldn't it list all the assumptions that the curve is based on. Like 70.220.140.142 pointed out "The aggregation of individual demand curves to derive a market demand curve is dependent on the assumption that each consumer is an idependent, idiosyncratic decision maker." One of the assuptions that I am aware of the is "all things being equal" line. Which would cover my good buddy 70.220.140.142's problem. Since nothing changes for each decision maker (all things being equal) the feed back loop he is concerned about would be eliminated. This is just one assumption that puts this curve (and the rest of the economics in my opinion) in the relm of reflexolog or divination by chicken bones. It would be nice to have a list assuptions front and centre so that readers can clearly see how far from reality the demand curve sits. Thank you.207.161.144.90 (talk) 02:46, 11 February 2014 (UTC) bob stuf

I agree that all assumptions should be listed as an essential part of the article. I disagree with your characterization of supply and demand as being unscientific as inaccurate because this is a fundamental principle of economics that is easily verifiable. A misunderstanding of supply and demand is that when it was formulated under classical economics there was a very low rate of technological progress, so classical supply and demand was static. Since the Industrial Revolution the more appropriate model is that of supply and demand shifts.Phmoreno (talk) 03:20, 11 February 2014 (UTC)

Verifiable!? Maybe I'm wrong, but I don't think you can plot two points on a single persons demand curve let alone verify shifts, aggregates, or any other derived behaviour. I'll make my point with an analogistic tale, the basis of all economics and tarot card discussions. A man walks into a store to buy milk where a cashier is eagerly waiting with pen in hand to plot a demand curve for this customer. The price of milk is set to $3.00 and the customer purchases one carton. There's one point for the curve, Q=1= f(p = $3.00). Huzzah!! The cashier lowers the price to $2.00, but the customer already has enough milk, so the next point cannot be determined. The cashier also realizes that even the price change itself is likely to influence the customers decision. If the price for milk changes from $3 to $2 the customer will react differently than say the price changes from $50 to $2. Even if this is ignored, waiting till tomorrow will likely add many other unknown factors that will effect the customer's decision. Luckly in this tale of fantasy the cashier has a time machine. He sets his time machine back 10 minutes, resetting the universe. All things are now assumed to be equal and the cashier is confident that the next plot point will be achieved. Oh no! in the excitement of time travel the cashier forgets to change the price! Too late the customer is here! Fumbling around the cooler, the customer is curiously taking longer this time. Eventually he walks up to the cashier and buys 2 cartons of milk. WTF!? Q={1,2} = f(p = $3.00)?? What happened? We can explain by looking at the thoughts of the customer. He is wondering if his nephews are going to stop by after they are done tobogganing. Sometimes they do and sometimes they don't. If they do stop by then he will make them hot chocolate, where he already has the chocolate, but he would need another carton of milk. When he gets to the cooler he makes a random decision by flipping a coin. The first time it landed on tails, the second time it landed on heads. Thats' too bad for plotting the demand curve. With this customer, at this point in time, at p=$3 sometimes Q =1 sometimes Q = 2. If only physics accepted analogical evidence, we could publish a paper on how the universe is a non-deterministic state machine where random events are truly random. It's obvious that this facetious argument can be countered by another yarn of fantasy. This time maybe with magical wizards instead of time machines, but this is the nature of economics, not science. Maybe its just me but I think an honest discussion would remind people (especially new commers) that economics is the use of calculus to determine how many angels can dance on the head of a pin without proving there is a pin, let alone angels that can dance. For this article, a list of assumptions would be that reminder. Furthermore, if you read the other discussions on this page they make reference to many assumptions that are not in the actual wiki-page. I hope that the moderator can see past my total contempt for this subject and include them. That is assuming of course that a consistent list exists. It maybe the case that the assumptions change from author to author depending on what idea they are trying to convince the reader of. The text book I have does not list these assumptions, nor does it ever state that the demand curve is purely theoretical and that none exists in this reality.207.161.144.90 (talk) 20:05, 11 February 2014 (UTC) bob stuf

Its a concept, not a formula. And while an individual merchant may not be able to determine a supply and demand curve, commodity markets and marketing departments do it routinely. The good forecasters survive, the poor ones go out of business.Phmoreno (talk) 01:11, 8 July 2014 (UTC)