Princ ch05-presentation

Editor's Notes

• #2: The elasticity chapter in most principles textbooks is fairly technical, and is not always students’ favorite. This PowerPoint chapter contains several special features designed to engage and motivate students to learn this important material. First, we consider a scenario in which students face a business decision – whether to raise the price of a service they sell. This scenario is used to illustrate the effects of raising price on number of units sold and on revenue, which students immediately recognize as critical to the business decision. Second, instead of merely listing the determinants of elasticity, students are asked to think about some concrete examples and deduce from each one a lesson about the determinants of elasticity. Third, instead of putting the applications at the end of the chapter (as in the textbook), this PowerPoint includes one of them immediately after the section on price elasticity of demand. This helps break up what would otherwise be a long stretch of theory. Please be assured that this PowerPoint presentation is, nonetheless, very consistent with the textbook’s approach.
• #4: We will follow this scenario throughout the first section of this chapter (the section on price elasticity of demand) to illustrate and motivate several important concepts, such as the impact of price changes on sales and revenue.
• #5: Here, Q d and Q s are short for quantity demanded and quantity supplied, as in the PowerPoint for Chapter 4.
• #8: It might be worth explaining to your students that “P and Q move in opposite directions” means that the percentage change in Q and the percentage change in P will have opposite signs, thus implying a negative price elasticity. To be consistent with the text, the last statement in the green box says that we will report all price elasticities as positive numbers. It might be slightly more accurate to say that we will report all elasticities as non-negative numbers: we want to allow for the (admittedly rare) case of zero elasticity.
• #12: These calculations are based on the example shown a few slides back: points A and B on the website demand curve.
• #15: In essence, the textbook says “Here are the determinants of elasticity. The first one is availability of close substitutes. Here’s an example….” That’s great for a textbook. For teaching, I’ve found a different approach to be far more effective: having students deduce the general lessons from specific examples they can figure out using common sense. This is the approach on the next few slides. Also, see notes on the next slide for a good suggestion.
• #16: Suggestion: For each of these examples, display the slide title (which lists the two goods) and the first two lines of text (which ask which good experiences the biggest drop in demand in response to a 20% price increase). Give your students a quiet minute to formulate their answers. Then, ask for volunteers.
• #17: You might need to clarify the nature of this thought experiment. Here, we look at two alternate scenarios. In the first, the price of blue jeans (and no other clothing) rises by 20%, and we observe the percentage decrease in quantity of blue jeans demanded. In the second scenario, the price of all clothing rises by 20%, and we observe the percentage decrease in demand for all clothing.
• #20: This slide is a convenience for your students, and replicates a similar table from the text. If you’re pressed for time, it is probably safe to omit this slide from your presentation.
• #21: Economists classify demand curves according to their elasticity. The next 5 slides present the five different classifications, from least to most elastic.
• #22: If Q doesn’t change, then the percentage change in Q equals zero, and thus elasticity equals zero. It is hard to think of a good for which the price elasticity of demand is literally zero. Take insulin, for example. A sufficiently large price increase would probably reduce demand for insulin a little, particularly among people with very low incomes and no health insurance. However, if elasticity is very close to zero, then the demand curve is almost vertical. In such cases, the convenience of modeling demand as perfectly inelastic probably outweighs the cost of being slightly inaccurate.
• #23: An example: student demand for textbooks that their professors have required for their courses. Here, it’s a little more clear that elasticity would be small, but not zero. At a high enough price, some students will not buy their books, but instead will share with a friend, or try to find them in the library, or just take copious notes in class. Another example: gasoline in the short run.
• #24: This is the intermediate case: the demand curve is neither relatively steep nor relatively flat. Buyers are neither relatively price-sensitive nor relatively insensitive to price. This is also the case where price changes have no effect on revenue.
• #25: A good example here would be Rice Krispies, or nearly anything with readily available substitutes. An elastic demand curve is flatter than a unit elastic demand curve (which itself is flatter than an inelastic demand curve).
• #26: “ Extreme price sensitivity” means the tiniest price increase causes demand to fall to zero. “ Q changes by any %” – when the D curve is horizontal, quantity cannot be determined from price. Consumers might demand Q1 units one month, Q2 units another month, and some other quantity later. Q can change by any amount, but P always “changes by 0%” (i.e. doesn’t change). If perfectly inelastic is one extreme, this (perfectly elastic) is the other. Here’s a good real-world example of a perfectly elastic demand curve, which foreshadows an upcoming chapter on firms in competitive markets. Suppose you run a small family farm in Iowa. Your main crop is wheat. The demand curve in this market is downward-sloping, and the market demand and supply curves determine the price of wheat. Suppose that price is $5/bushel. Now consider the demand curve facing you, the individual wheat farmer. If you charge a price of $5, you can sell as much or as little as you want. If you charge a price even just a little higher than $5, demand for YOUR wheat will fall to zero: Buyers would not be willing to pay you more than $5 when they could get the same wheat elsewhere for $5. Similarly, if you drop your price below $5, then demand for YOUR wheat will become enormous (not literally infinite, but “almost infinite”): if other wheat farmers are charging $5 and you charge less, then EVERY buyer will want to buy wheat from you. Why is the demand curve facing an individual producer perfectly elastic? Recall that elasticity is greater when lots of close substitutes are available. In this case, you are selling a product that has many perfect substitutes: the wheat sold by every other farmer is a perfect substitute for the wheat you sell.
• #27: The material on this slide is not used anywhere else in the textbook. Therefore, if you are pressed for time and looking for things to cut, you might consider cutting this slide. (Note that this is my personal recommendation and is not necessarily the official position of Greg Mankiw or Thomson/South-Western.) Due to space limitations, this slide uses “E” as an abbreviation for elasticity, or more specifically, the price e lasticity of demand, and the slide omits the analysis of revenue along the demand curve. Calculations of percentage changes use the midpoint method. (This is why the increase from Q=0 to Q=20 is 200% rather than infinity.) As you move down a linear demand curve, the slope (the ratio of the absolute change in P to that in Q) remains constant: From the point (0, $30) to the point (20, $20), the “rise” equals -$10, the “run” equals +20, so the slope equals -1/2 or -0.5. From the point (40, $10) to the point (60, $0), the “rise” again equals -$10, the “run” equals +20, and the slope again equals -0.5. However, the percentage changes in these variables do not remain constant, as shown by the different colored elasticity calculations that appear on the slide. The lesson here is that elasticity falls as you move downward & rightward along a linear demand curve.
• #28: We return to our scenario. It’s not hard for students to imagine being in this position – running their own business and trying to decide whether to raise the price. To most of your students, it should be clear that making the best possible decision would require information about the likely effects of the price increase on revenue. That is why elasticity is so helpful, as we will now see….
• #30: In the “Normal” view (edit mode), the labels over the graph look cluttered, like they’re on top of each other. This is not a mistake – in “Slide Show” mode (presentation mode), they will be fine – try it! Point out to students that the area (outlined in blue) representing lost revenue due to lower Q is larger than the area (outlined in yellow) representing increased revenue due to higher P. Hence, the net effect is a fall in revenue.
• #32: Again, the slide appears cluttered in “Normal” view (edit mode), but everything is fine when displayed in “Slide Show” mode (presentation mode). Point out to students that the area representing lost revenue due to lower Q is smaller than the area representing increased revenue due to higher P. Hence, the net effect is an increase in revenue. The knife-edge case, not shown here but perhaps worth mentioning in class, is unit-elastic demand. In that case, an increase in price leaves revenue unchanged: the increase in revenue from higher P exactly offsets the lost revenue due to lower Q.
• #33: These problems, perhaps similar to those you might ask on an exam, are complex in that they test several skills at once: Students must determine whether demand for each good is elastic or inelastic, and they must determine the impact of a price change on revenue/expenditure. So far, we’ve been talking about how elasticity determines the effects of an increase in P on revenue. Part (b) asks your students to determine the effects of a decrease in P.
• #35: The first part of the explanation discusses the opposing effects on revenue; its purpose is to clarify the effects of a price decrease on revenue, as we have previously only discussed the effects of a price increase.
• #36: In the textbook, this application appears near the end of the chapter, and you can easily move these slides to the end if you wish to teach things in the same order as the book. However, I encourage you to consider teaching this application right here - immediately after the section on price elasticity of demand. It is safe to do so, as this application only requires knowledge of price elasticity of demand. Also, putting the application here breaks up what would otherwise be a very long section of theory with a real-world example that most students find very interesting. Knowing elasticity helps us understand what might otherwise be a counter-intuitive result (that drug interdiction increases drug-related crime rather than reducing it).
• #37: By the time all elements have appeared on the screen, the slide will look kind of busy. I think this is okay, because the elements appear on the screen one by one, so students have time to absorb each one before the next one appears. However, if you’d rather strip the slide down a bit, here’s a suggestion: in “Normal” view (which is used to edit slides), you can delete the boxes that represent the initial and new values of drug-related crime, and the accompanying captions. Then, when presenting this slide in class, simply point out (with your mouse cursor, a laser pointer, or even your arms and hands) the areas that represent the initial and new values of drug-related crime.
• #39: Most everything in the “price elasticity of supply” section corresponds to analogous concepts from the “price elasticity of demand” section. So, it is probably safe to move through this section more quickly.
• #41: Economists classify supply curves according to their elasticity. The next 5 slides present the different classifications, from least to most elastic.
• #47: This section is not perfectly analogous to the section on the determinants of the price elasticity of demand, but it’s similar enough that you can probably cover it more quickly and with less hand-holding.
• #48: This is one of the “Problems and Applications” at the end of the chapter.
• #49: In this slide and the next, the initial price and quantity and the two demand curves are the same. The only difference is the elasticity of supply and slope of the supply curve. [The D curve shifts to the right, but not in a parallel fashion: at each price, quantity demanded is twice as high, so the new D curve will be flatter than the initial one.] In the text box containing the verbal explanation, “bigger impact” is shorthand for “bigger percentage impact” or “bigger proportional impact.”
• #51: This graph replicates the one in Figure 6. Note: The graph here is not quite drawn to scale. When the price rises from $3 to $4 (a 29% increase, using the midpoint method), quantity rises from 100 to 120 (or 67%). Because 67% > 29%, price elasticity of supply is greater than one. When the price rises from $12 to $15 (22%), quantity rises from 500 to 525 (about 5%), so price elasticity of supply is less than one. The way I like to explain this is as follows: When output is very low, it is relatively easy for firms to increase output. They may have excess capacity, or they are not requiring full effort from their workers. Increasing output is not difficult, so it doesn’t take much of an increase in price to induce an increase in production. When output is very high, it is relatively expensive for firms to increase output further: there’s little or no excess capacity, they are already running their factories and machines at a high level of intensity. To increase output further, they might have to pay their workers overtime, and their machines experience more wear and tear and therefore require more repairs. So, at high levels of output, it takes a much larger price increase to make firms willing to increase output further. Eventually, firms bump up against their capacity constraints, and simply cannot increase output in response to further price increases. Of course, all of this applies to the short run. In the long run, firms can build more factories, and (depending on the market structure) new firms can enter the market.
• #52: This topic and the next one (cross-price elasticity) do not appear anywhere else in the book. Instructors who are pressed for time may consider cutting these topics. (This is merely my suggestion, not the official position of Greg Mankiw or Thomson/South-Western.)