Week 6 FAQs


Wednesday July 19, 2023 at 10:02 AM

Hi everyone!

Here are a few common questions and comments from your weekly check-ins and problem set 6:

What do you do when lines are parallel?

Often you’ll be working with supply and demand equations that intersect beyond the basic Desmos window (i.e. between -10 and 10). In question 4 in Problem Set 7, for instance, the lines cross way out where x = 2,500, but y is only 0.75. Seeing that in Desmos is tricky—it can be done, but you have to zoom out really far. What I typically do in situations like this is draw (on paper) an oversimplified and incorrect picture. If you look at the answer key for 7.4, you’ll see that the demand line is like a normal 45º angle, which is fine (since the x-axis is super condensed). If I did it on paper or on a whiteboard, I’d do the same thing—just draw a downward sloping line (for demand; draw an upward sloping line for supply). I then write the coordinates where the lines cross so that I can figure out the DWL and other rectangles and triangles. The super simple diagram won’t be to scale, but it’ll help show the different sections—you just have to keep track of the intersection points (which you can get with Desmos or algebra)

More details

See this question from the FAQs for week 3 for more about zooming with Desmos.

Is deadweight loss always shaped like a triangle?

Pretty much, yeah, it’ll be a triangle, since you have lines sloping down and up at different angles. It won’t ever overlap with other sections of the graph—part of DWL can’t be included in consumer/producer surplus or tax revenue, since each of those areas stand alone. Look for a triangle near the intersection of supply/demand and shifted supply/demand and you’ll find the DWL.

What does deadweight loss mean in real life?

In practical terms, DWL means that some people can no longer participate in a market. If the existing market price for a thing is $5, and a tax raises that to $7, then everyone who’d be willing to pay $5.50 or $6 or $6.99 for the thing are priced out and won’t buy it, which means the company won’t get their money. All that potential money from those lower-value transactions disappears.

For people that can still afford the thing, they’ll feel worse about it. If someone was willing to pay $7.50 and originally only had to pay $5, they’d get $2.50 in surplus, or what I like to call “good deal points”—it feels like you’re saving a bunch of money or getting a good deal. If the tax makes the price go up to $7, they’d only get $0.50 in surplus, which is far fewer “good deal points.” They can still afford and buy the thing, but it’s less of a good deal, and you feel worse about it.

Why are positive externalities considered market failures?

Positive externalities are a market “failure” because of the definition of a market failure. Markets “fail” when they don’t account for the full price of something. With negative externalities, they don’t take into account the price of downstream consequences, like pollution. With positive externalities, markets fail because they again don’t take into account the positive downstream benefits (like basic scientific research or editing a wikipedia page), so it generally leads to free riding and underprovision. Positive externalities are thus failures (even though they’re good!) because the market doesn’t fully account for them.

How do you know how to identify different market failures? There seems to be a lot of overlap…

With many market failures, there are actually multiple ways to describe them, so it’s fine if you struggled to settle on just one when filling out the table in problem set 6. For instance, positive externalities are a market failure (even though they’re a good thing), but they inherently lead to free riding and underprovision, which also happens with public goods, which is also a market failure. There’s a lot of overlap.