# 3.2 – Supply

## Learning Objectives

• Define the quantity supplied of a good or service and illustrate it using a supply schedule and a supply curve.
• Distinguish between the following pairs of concepts: supply and quantity supplied, supply schedule and supply curve, movement along and shift in a supply curve.
• Identify supply shifters and determine whether a change in a supply shifter causes the supply curve to shift to the right or to the left.

What determines the quantity of a good or service sellers are willing to offer for sale? Price is one factor; ceteris paribus, a higher price is likely to induce sellers to offer a greater quantity of a good or service. Production cost is another determinant of supply. Variables that affect production cost include the prices of factors used to produce the good or service, returns from alternative activities, technology, the expectations of sellers, and natural events such as weather changes. Still another factor affecting the quantity of a good that will be offered for sale is the number of sellers—the greater the number of sellers of a particular good or service, the greater will be the quantity offered at any price per time period.

## Price and the Supply Curve

The quantity supplied of a good or service is the quantity sellers are willing to sell at a particular price during a particular period, all other things unchanged. Ceteris paribus, the receipt of a higher price increases profits and induces sellers to increase the quantity they supply.

In general, when there are many sellers of a good, an increase in price results in an increase in quantity supplied, and this relationship is often referred to as the law of supply. We will see, though, through our exploration of microeconomics, that there are a number of exceptions to this relationship. There are cases in which a higher price will not induce an increase in quantity supplied. Goods that cannot be produced, such as additional land on the corner of Park Avenue and 56th Street in Manhattan, are fixed in supply—a higher price cannot induce an increase in the quantity supplied. There are even cases, which we investigate in microeconomic analysis, in which a higher price induces a reduction in the quantity supplied.

Generally speaking, however, when there are many sellers of a good, an increase in price results in a greater quantity supplied. The relationship between price and quantity supplied is suggested in a supply schedule, a table that shows quantities supplied at different prices during a particular period, all other things unchanged. Figure 3.2a “A Supply Schedule and a Supply Curve” gives a supply schedule for the quantities of coffee that will be supplied per month at various prices, ceteris paribus. At a price of \$4 per pound, for example, producers are willing to supply 15 million pounds of coffee per month. A higher price, say \$6 per pound, induces sellers to supply a greater quantity—25 million pounds of coffee per month.

Figure 3.2a A Supply Schedule and a Supply Curve Textual Version

Figure 3.2a: The vertical axis is price per pound (\$)and the horizontal axis is quantity of coffee supplied per month. The supply curve slopes upward from left to right. The values given here suggest a positive relationship between price and quantity supplied. Table 3.2a A Supply Schedule and a Supply Curve contains data from Figure 3.2a.

Table 3.2a A Supply Schedule and a Supply Curve
Price per pound (\$) Quantity supplied per month (millions per pounds of coffee per month)
4 15
5 20
6 25
7 30
8 35
9 40

The supply schedule shows the quantity of coffee that will be supplied in the United States each month at particular prices, all other things unchanged. The same information is given graphically in the supply curve. The values given here suggest a positive relationship between price and quantity supplied. A supply curve is a graphical representation of a supply schedule. It shows the relationship between price and quantity supplied during a particular period, all other things unchanged. Because the relationship between price and quantity supplied is generally positive, supply curves are generally upward sloping. The supply curve for coffee in Figure 3.2a “A Supply Schedule and a Supply Curve” shows graphically the values given in the supply schedule.

A change in price causes a movement along the supply curve; such a movement is called a change in quantity supplied. As is the case with a change in quantity demanded, a change in quantity supplied does not shift the supply curve. By definition, it is a movement along the supply curve. For example, if the price rises from \$6 per pound to \$7 per pound, the quantity supplied rises from 25 million pounds per month to 30 million pounds per month. That’s a movement from point A to point B along the supply curve in Figure 3.2a “A Supply Schedule and a Supply Curve”.

## Changes in Supply

When we draw a supply curve, we assume that other variables that affect the willingness of sellers to supply a good or service are unchanged. It follows that a change in any of those variables will cause a change in supply, which is a shift in the supply curve. A change that increases the quantity of a good or service supplied at each price shifts the supply curve to the right. Suppose, for example, that the price of fertilizer falls. That will reduce the cost of producing coffee and thus increase the quantity of coffee producers will offer for sale at each price. The supply schedule in Figure 3.2b “An Increase in Supply” shows an increase in the quantity of coffee supplied at each price. Table 3.9 An Increase in Supply details data from Figure 3.9 An Increase in Supply.

If there is a change in supply that increases the quantity supplied at each price, as is the case in the supply schedule here, the supply curve shifts to the right from S1 to S2. At a price of \$6 per pound, for example, the quantity supplied rises from the previous level of 25 million pounds per month on supply curve S1 (point A) to 35 million pounds per month on supply curve S2 (point A′).

Figure 3.2b An Increase in Supply Textual Version

The vertical axis is price per pound and the horizontal axis is quantity of coffee per month. The supply curve slopes upward left to right. There is a change in supply that increases the quantity supplied at each price causing the supply curve shifts to the right from original supply curve (S1) to the new supply curve S2. At a price of \$6 per pound, for example, the quantity supplied rises from the previous level of 25 million pounds per month on supply curve S1 (point A) to 35 million pounds per month on supply curve S2 (point A′). Table 3.9 An Increase in Supply contains data from Figure 3.9.

Table 3.2b An Increase in Supply
Price (\$) Old quantity supplied (S1) New quantity supplied (S2)
4 15 25
5 20 30
6 25 35
7 30 40
8 35 45
9 40 50

We show that increase graphically as a shift in the supply curve from S1 to S2. We see that the quantity supplied at each price increases by 10 million pounds of coffee per month. At point A on the original supply curve S1, for example, 25 million pounds of coffee per month are supplied at a price of \$6 per pound. After the increase in supply, 35 million pounds per month are supplied at the same price (point A′ on curve S2).

An event that reduces the quantity supplied at each price shifts the supply curve to the left. An increase in production costs and excessive rain that reduces the yields from coffee plants are examples of events that might reduce supply. Figure 3.2c “A Reduction in Supply” shows a reduction in the supply of coffee. We see in the supply schedule that the quantity of coffee supplied falls by 10 million pounds of coffee per month at each price. The supply curve thus shifts from S1 to S3.

Figure 3.2c A Reduction in Supply Textual Version

Figure 3.2c Graph: The vertical axis is price per pound (\$) and horizontal axis is quantity (millions of pounds of coffee per month). The supply curve slopes upwards left to right. A change in supply that reduces the quantity supplied at each price shifts the original supply curve (S1) to the left to the new supply curve (S3). At a price of \$6 per pound, for example, the original quantity supplied was 25 million pounds of coffee per month (point A). With a new supply curve S3, the quantity supplied at that price falls to 15 million pounds of coffee per month (point A″).Table 3.2c A Reduction in Supply contains data from Figure 3.2c.

Table 3.2c A Reduction in Supply
Price (\$) Old quantity supplied New quantity supplied
4 15 5
5 20 10
6 25 15
7 30 20
8 35 25
9 40 30

Figure 3.2c A Reduction in Supply. A change in supply that reduces the quantity supplied at each price shifts the supply curve to the left. At a price of \$6 per pound, for example, the original quantity supplied was 25 million pounds of coffee per month (point A). With a new supply curve S3, the quantity supplied at that price falls to 15 million pounds of coffee per month (point A″).

A variable that can change the quantity of a good or service supplied at each price is called a supply shifter. Supply shifters include:

1. prices of factors of production
2. returns from alternative activities
3. technology
4. seller expectations
5. natural events
6. the number of sellers.

When these other variables change, the all-other-things-unchanged conditions behind the original supply curve no longer hold. Let us look at each of the supply shifters.

## Prices of Factors of Production

A change in the price of labour or some other factor of production will change the cost of producing any given quantity of the good or service. This change in the cost of production will change the quantity that suppliers are willing to offer at any price. An increase in factor prices should decrease the quantity suppliers will offer at any price, shifting the supply curve to the left. A reduction in factor prices increases the quantity suppliers will offer at any price, shifting the supply curve to the right.

Suppose coffee growers must pay a higher wage to the workers they hire to harvest coffee or must pay more for fertilizer. Such increases in production cost will cause them to produce a smaller quantity at each price, shifting the supply curve for coffee to the left. A reduction in any of these costs increases supply, shifting the supply curve to the right.

## Returns from Alternative Activities

To produce one good or service means forgoing the production of another. The concept of opportunity cost in economics suggests that the value of the activity forgone is the opportunity cost of the activity chosen; this cost should affect supply. For example, one opportunity cost of producing eggs is not selling chickens. An increase in the price people are willing to pay for fresh chicken would make it more profitable to sell chickens and would thus increase the opportunity cost of producing eggs. It would shift the supply curve for eggs to the left, reflecting a decrease in supply.

## Technology

A change in technology alters the combinations of inputs or the types of inputs required in the production process. An improvement in technology usually means that fewer and/or less costly inputs are needed. If the cost of production is lower, the profits available at a given price will increase, and producers will produce more. With more produced at every price, the supply curve will shift to the right, meaning an increase in supply.

Impressive technological changes have occurred in the computer industry in recent years. Computers are much smaller and are far more powerful than they were only a few years ago—and they are much cheaper to produce. The result has been a huge increase in the supply of computers, shifting the supply curve to the right.

While we usually think of technology as enhancing production, declines in production due to problems in technology are also possible. Outlawing the use of certain equipment without pollution-control devices has increased the cost of production for many goods and services, thereby reducing profits available at any price and shifting these supply curves to the left.

## Seller Expectations

All supply curves are based in part on seller expectations about future market conditions. Many decisions about production and selling are typically made long before a product is ready for sale. Those decisions necessarily depend on expectations. Changes in seller expectations can have important effects on price and quantity.

Consider, for example, the owners of oil deposits. Oil pumped out of the ground and used today will be unavailable in the future. If a change in the international political climate leads many owners to expect that oil prices will rise in the future, they may decide to leave their oil in the ground, planning to sell it later when the price is higher. Thus, there will be a decrease in supply; the supply curve for oil will shift to the left.

## Natural Events

Storms, insect infestations, and drought affect agricultural production and thus the supply of agricultural goods. If something destroys a substantial part of an agricultural crop, the supply curve will shift to the left. The terrible cyclone that killed more than 50,000 people in Myanmar in 2008 also destroyed some of the country’s prime rice growing land. That shifted the supply curve for rice to the left. If there is an unusually good harvest, the supply curve will shift to the right.

## The Number of Sellers

The supply curve for an industry, such as coffee, includes all the sellers in the industry. A change in the number of sellers in an industry changes the quantity available at each price and thus changes supply. An increase in the number of sellers supplying a good or service shifts the supply curve to the right; a reduction in the number of sellers shifts the supply curve to the left.

The market for cellular phone service has been affected by an increase in the number of firms offering the service. Over the past decade, new cellular phone companies emerged, shifting the supply curve for cellular phone service to the right.

There are two special things to note about supply curves. First, it is important to distinguish carefully between changes in supply and changes in quantity supplied. A change in supply results from a change in a supply shifter and implies a shift of the supply curve to the right or left. A change in price produces a change in quantity supplied and induces a movement along the supply curve. A change in price does not shift the supply curve.

The second caution relates to the interpretation of increases and decreases in supply. Notice that in Figure 3.2b “An Increase in Supply” an increase in supply is shown as a shift of the supply curve to the right; the curve shifts in the direction of increasing quantity with respect to the horizontal axis. In Figure 3.2c “A Reduction in Supply” a reduction in supply is shown as a shift of the supply curve to the left; the curve shifts in the direction of decreasing quantity with respect to the horizontal axis.

Because the supply curve is upward sloping, a shift to the right produces a new curve that in a sense lies “below” the original curve. Students sometimes make the mistake of thinking of such a shift as a shift “down” and therefore as a reduction in supply. Similarly, it is easy to make the mistake of showing an increase in supply with a new curve that lies “above” the original curve. But that is a reduction in supply!

To avoid such errors, focus on the fact that an increase in supply is an increase in the quantity supplied at each price and shifts the supply curve in the direction of increased quantity on the horizontal axis. Similarly, a reduction in supply is a reduction in the quantity supplied at each price and shifts the supply curve in the direction of a lower quantity on the horizontal axis.

## Key Takeaways

• The quantity supplied of a good or service is the quantity sellers are willing to sell at a particular price during a particular period, all other things unchanged.
• A supply schedule shows the quantities supplied at different prices during a particular period, all other things unchanged. A supply curve shows this same information graphically.
• A change in the price of a good or service causes a change in the quantity supplied—a movement along the supply curve.
• A change in a supply shifter causes a change in supply, which is shown as a shift of the supply curve. Supply shifters include prices of factors of production, returns from alternative activities, technology, seller expectations, natural events, and the number of sellers.
• An increase in supply is shown as a shift to the right of a supply curve; a decrease in supply is shown as a shift to the left.

## Try It!

If all other things are unchanged, what happens to the supply curve for DVD rentals if there is (a) an increase in wages paid to DVD rental store clerks, (b) an increase in the price of DVD rentals, or (c) an increase in the number of DVD rental stores? Draw a graph that shows what happens to the supply curve in each circumstance. The supply curve can shift to the left or to the right, or stay where it is. Remember to label the axes and curves, and remember to specify the time period (e.g., “DVDs rented per week”).

Figure 3.2e Textual Version

Figure 3.2e depicts the changes in supply curve in 4 scenarios: panel A, panel B, and panel C. The graphs all have Price per DVD rental on the vertical axis and the Quantity of DVD rentals per period on the horizontal axis. The supply curve is linear sloping upwards left to right and the original curve is denoted by S1 and change in the Supply curve by S2.

Panel A: DVD rental store clerks are a factor of production in the DVD rental market. An increase in their wages raises the cost of production, thereby causing the supply curve of DVD rentals to shift to the left. (Caution: It is possible that you thought of the wage increase as an increase in income, a demand shifter, that would lead to an increase in demand, but this would be incorrect. The question refers only to wages of DVD rental store clerks. They may rent some DVD, but their impact on total demand would be negligible. Besides, we have no information on what has happened overall to incomes of people who rent DVDs. We do know, however, that the cost of a factor of production, which is a supply shifter, increased.)

Panel B: An increase in the price of DVD rentals does not shift the supply curve at all; rather, it corresponds to a movement upward to the right along the supply curve. At a higher price of P2 instead of P1, a greater quantity of DVD rentals, say Q2 instead of Q1, will be supplied.

Panel C: An increase in the number of stores renting DVDs will cause the supply curve to shift to the right.

## Case in Point: The Monks of St. Benedict’s Get Out of the Egg Business

It was cookies that lured the monks of St. Benedict’s out of the egg business, and now private retreat sponsorship is luring them away from cookies.St. Benedict’s is a Benedictine monastery, nestled on a ranch high in the Colorado Rockies, about 20 miles down the road from Aspen. The monastery’s 15 monks operate the ranch to support themselves and to provide help for poor people in the area. They lease out about 3,500 acres of their land to cattle and sheep grazers, produce cookies, and sponsor private retreats. They used to produce eggs.

Attracted by potential profits and the peaceful nature of the work, the monks went into the egg business in 1967. They had 10,000 chickens producing their Monastery Eggs brand. For a while, business was good. Very good. Then, in the late 1970s, the price of chicken feed started to rise rapidly.

“When we started in the business, we were paying \$60 to \$80 a ton for feed—delivered,” recalls the monastery’s abbot, Father Joseph Boyle. “By the late 1970s, our cost had more than doubled. We were paying \$160 to \$200 a ton. That really hurt, because feed represents a large part of the cost of producing eggs.”

The monks adjusted to the blow. “When grain prices were lower, we’d pull a hen off for a few weeks to molt, then return her to laying. After grain prices went up, it was 12 months of laying and into the soup pot,” Father Joseph says.

Grain prices continued to rise in the 1980s and increased the costs of production for all egg producers. It caused the supply of eggs to fall. Demand fell at the same time, as Americans worried about the cholesterol in eggs. Times got tougher in the egg business.

“We were still making money in the financial sense,” Father Joseph says. “But we tried an experiment in 1985 producing cookies, and it was a success. We finally decided that devoting our time and energy to the cookies would pay off better than the egg business, so we quit the egg business in 1986.”

The mail-order cookie business was good to the monks. They sold 200,000 ounces of Monastery Cookies in 1987.

By 1998, however, they had limited their production of cookies, selling only locally and to gift shops. Since 2000, they have switched to “providing private retreats for individuals and groups—about 40 people per month,” according to Brother Charles.

The monks’ calculation of their opportunity costs revealed that they would earn a higher return through sponsorship of private retreats than in either cookies or eggs. This projection has proved correct.

And there is another advantage as well.

“The chickens didn’t stop laying eggs on Sunday,” Father Joseph chuckles. “When we shifted to cookies we could take Sundays off. We weren’t hemmed in the way we were with the chickens.” The move to providing retreats is even better in this regard. Since guests provide their own meals, most of the monastery’s effort goes into planning and scheduling, which frees up even more of their time for other worldly as well as spiritual pursuits.

Source: Personal interviews.