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## Consumer and producer surplus

# Equilibrium, allocative efficiency and total surplus

AP.MICRO:

MKT‑4 (EU)

, MKT‑4.A.4 (EK)

, MKT‑4.A.5 (EK)

## Video transcript

- [Instructor] What we're
going to do in this video is think about the market for chocolate and we're going to think about
supply and demand curves, but we're going to get an intuition for them in a slightly different way. In particular for the demand curve we will think about the
idea of marginal benefit. Now marginal benefit, when
we're talking about margin it's really thinking about, well, what happens on the increment? What happens for each
little extra that you do? So this is saying, what
is the benefit that I get if I get a little bit more
of, in this case, chocolate? Well, from the market's point of view, imagine if there was no chocolate, but there's people in the market who crave chocolate, who dream of chocolate. If all of a sudden they were able to get their hands on some chocolate, they would get a huge benefit for that incremental amount of chocolate. Maybe for these folks, their benefit, which we could quantify
as, in terms of dollars, maybe their benefit is
50 dollars per pound. One way to think about
it, they'd be willing to pay 50 dollars because
they get that much benefit, or if they paid less than 50 dollars, let's say they paid 10 dollars for it, well then they're getting
40 dollars of extra benefit a kind of surplus of benefit
from being able to get it at a price lower than
their marginal benefit. But then let's say more
chocolate becomes available. People still like it but some
of that really deep need, that deep addiction for
chocolate has been satiated, and so the marginal benefit
tends for, in most markets, the marginal benefit tends to
go down as quantity increases. One way to think about it,
that first initial amount of quantity if you, so
we have some small amount of quantity right over here,
I'll say delta quantity. That first quantity, if
you multiply it times the marginal benefit, well
that gives you an area roughly of a rectangle
like this, it's not quite rectangular at the top,
it's more of a trapezoid if this is downward sloping, but you could approximate it as a rectangle. But either way, the area right over here, the area under the marginal benefit curve, you could think about this as, well, that's just the benefit
that the market is getting from consuming this
chocolate in this case. And so let's just continue on this trend. If there's more and more chocolate the market will get benefit from it, but people aren't as
excited about it anymore. They're saying, oh, well,
the chocolate's around, yeah, it'd be nice to
have a little bit more, but I don't need so much more. And at some point people might be all chocolated out, and they have maybe even zero marginal benefit from that incremental amount of chocolate, chocolate has filled up the town, there's nowhere to actually put it. Now that won't always be the case, you might go someplace like that, but either way you think about it we would view this as our
marginal benefit curve. And notice, this is exactly the same as a demand curve in the
market for chocolate. We have plotted price versus quantity in the market for
chocolate, but we've thought about it in terms of marginal benefit. Now on the supply side
there's a related idea, we're going to think about
marginal cost, marginal cost. So let's say at first there's no chocolate being produced in this market. And a savvy entrepreneur says, hey, I know some folks who are
addicted to chocolate, they would get a lot of benefit from it, so I'm going to try to
produce some chocolate. And they look around and they find out, hey, there's actually a
derelict chocolate factory in town that no one is
using and it's surrounded by these wild cocoa bushes
that are perfect for chocolate. And there are some people in town who are actually unemployed, but they are amazing at producing chocolate. And so the first units of chocolate, the marginal cost to produce
it is actually quite low. But once you utilize those folks, you utilize that derelict factory, you utilize those free
cocoa bushes or whatever, cocoa trees, well then
you've got to plant new ones, you've got to train new employees, you've got to build a new factory. And so to produce that next increment, well that's going to cost
a little bit more and then a little bit more and
then a little bit more, which is the general
trend in most markets. Initially that first amount
you produce in as cheap a way, using the low hanging fruit, as possible, but then you've got to go up the tree, find higher and higher fruit,
maybe I'm mixing metaphors. But your cost, your marginal cost per unit goes higher and higher and higher. Now, what have we constructed here? Well you might say, hey, Sal,
that's a marginal cost curve. But once again, this
also could be viewed as the supply curve for
this particular market. Now what is happening at these low quantities right over here? Well, the cost of production is, let's say they produce
this delta Q amount, the cost of production would be the area right over here under
the marginal cost curve, that would be the cost of production. But they're able to sell it, the benefit to the market I should say, would be the total area
under this red curve, would be the benefit to the market, the total area under this curve. So if you have the total
benefit to the market, you take out the cost, then what you have in between these curves,
you could view this as a surplus, you could view this as a surplus of benefit right over here. So let me write this
down, this is surplus, and you won't hear this
term but I like to use it, because it makes it intuitive on what we're talking about,
this is surplus benefit. So as long as there's surplus benefit the suppliers are going to say, hey, wow, I can produce this cheaply, people have a, people get a lot of benefit for it, they'd be definitely willing to pay 10 dollars a pound wherever I am. If people are getting this much benefit, they're definitely going to be willing to pay 10 dollars for it. So I'm going to produce some, or actually I'm going to produce some
and I could even charge, I could charge anywhere
in between these areas. Maybe I could charge right over here and I get some of the benefit and then the consumers get some of the benefit. But then another maybe
entrepreneur realizes, hey, well, there's more benefit to be had in this market, so they keep producing, they keep producing as long
as there is benefit here, as long as the marginal benefit is higher than the marginal
cost, all the way until we get to that point right over here. Now what happens, what
happens right over here? Well we talked about
just supply and demand, we talk about that's an efficient price and efficient quantity,
but let's just think about, we said up until this point it makes sense to produce more and more and more. Even this increment, if we're
already at this quantity, it makes sense to produce
even a little bit more because you're going to have this cost, you're going to have this cost to incur, but then the market
could have all of this, the market could have all of this benefit, which is larger than the cost. And so you say, well, as long as I sell it for something in between we can split that surplus benefit, so to speak. But once these two lines intersect and we have the situation
where our marginal benefit, marginal benefit, is equal
to our marginal cost, well at that point there
is no surplus benefit now to be had, there's no really incentive to produce more than this. Beyond this point, your
incremental cost of production, your marginal cost is higher
than your marginal benefit. So, if you actually wanted
to give it to someone for their benefit, you
would be taking a loss. Or even if you just think
about the market itself, the society would be incurring more incremental cost per unit than they would be getting of benefit, so why even do it? And so this point right over here where these two lines,
these two curves intersect and we've talked about this
with just supply and demand, but when we think about it
in terms of marginal benefit and marginal cost, we
think about this quantity right over here, let's
just call it Q subzero, this quantity is considered allocatively efficient, allocatively efficient. Which is a very fancy word,
allocatively efficient. Why is that the case? Well, any other quantity
would not be efficient. For example, let's say
for some reason we were at this quantity right over
here, let's say Q, Q one. Well what happens at this
quantity right over here? Well, at this quantity, at
this quantity right over here, the marginal benefit is
higher than the marginal cost. Marginal benefit is greater
than the marginal cost. And so we're leaving a
bunch of stuff on the table, the market is leaving a
bunch of surplus benefit, you could say total surplus, on the table. And so this benefit that
the market could have had, but it does not get, this
is called a deadweight loss, deadweight loss, and we talk
about it in other videos. Remember, in the allocatively
efficient quantity we have this huge total surplus, which is the area under
the marginal benefit curve and above the marginal cost curve up until the point of intersection. But if you do a quantity less than that allocatively efficient quantity, your marginal benefit is higher than your marginal cost, and you are leaving, you are leaving all of this
total surplus on the table, regardless of how you would
have actually allocated it or distributed it between the
consumers or the producers. And what if you produced a quantity larger than the allocatively efficient quantity? Once again, very fancy word, so let's say that's Q two, what happens over here? Well, here you're able to take advantage of all of this surplus right over here, this total surplus right over here, but now you're creating negative surplus. So, in this part, now
all of this area shows a net negative benefit, or
a net I guess I should say a net cost that our market is incurring. And so this here it was
a deadweight loss because we were leaving stuff on the
table that we could have had. Here we're producing at
a cost that our market, not just our suppliers
are producing at a cost, the benefit our market is getting is less for each incremental unit, is less than, or is far less than the cost and so we are incurring a
net negative total surplus. And so this, too, would be considered, this, too, would be considered
a dead, let me write that in a color you can see, a deadweight loss. Deadweight loss we often
assume it was, hey, we're leaving some total
surplus on the table, but we also have
deadweight loss in the case where we are producing unnecessarily because the benefit is
less than the actual cost. And so whether our
marginal benefit is greater than our marginal cost, or in this case our marginal cost is greater
than our marginal benefit, we are going to produce deadweight
loss in either situation. And a properly functioning
market should be producing the quantity that
is allocatively efficient. In an ideal world and of
course all of our models assume a lot of assumptions to make
things a little bit cleaner, so we can do lines to
describe market behavior.