中文（繁體）
中文（简体）
English
DSE Economics | Price Controls and Quotas Lesson-Prep Guide (Tutor Edition) | Shortage, Surplus and Quota Arithmetic
Students can draw supply and demand yet mix up a rise in demand with a rise in quantity demanded the moment a ceiling appears. This lesson fixes one routine: find which side the control sits on, then read which quantity is stuck.
Objectives — Students can: (1) decide whether a ceiling or floor is above or below equilibrium and compute the shortage or surplus in units; (2) explain non-price competition and misallocation under a control; (3) compute the quantity cut from a quota and identify the price rise and deadweight loss.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Equilibrium | How to read equilibrium price and quantity | Where the curves cross |
| Demand vs quantity demanded | Which one moves when price changes | Quantity demanded, along the curve |
| Percentages | How far below 50 is 40 | 20% |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-10 min | Redraw equilibrium | Price 50, quantity 120 units | Both points marked |
| 10-24 min | Price ceiling | Below equilibrium: quantity demanded up, supplied down | Shortage of 50 units |
| 24-38 min | Price floor | Above equilibrium: supplied up, demanded down | Surplus of 45 units |
| 38-48 min | Quota | A cap turns supply vertical at the limit | 16.7% cut |
| 48-56 min | Deadweight loss | Blocked trades are the loss | Name the triangle |
| 56-60 min | Exit check | Three questions marked on the spot | All three right |
03 Teaching script (read it out)
Scene 1: A price ceiling only bites below equilibrium
Equilibrium is 50 with 120 units. A ceiling at 40 is 20% below equilibrium.
At that price quantity demanded rises to 140 while quantity supplied falls to 90, so the shortage is 140 minus 90 = 50 units, 35.7% of quantity demanded.
The price cannot rise to clear it, so competition moves off price: queues, ballots, rationing, black markets. That step is where explanation marks sit; saying shortage alone is half an answer.
At that price quantity demanded rises to 140 while quantity supplied falls to 90, so the shortage is 140 minus 90 = 50 units, 35.7% of quantity demanded.
The price cannot rise to clear it, so competition moves off price: queues, ballots, rationing, black markets. That step is where explanation marks sit; saying shortage alone is half an answer.
Ask students: what a ceiling above equilibrium achieves (nothing, the market simply settles at equilibrium).
Scene 2: A price floor only bites above equilibrium
The same market with a floor at 60, 20% above equilibrium: quantity supplied rises to 150 while quantity demanded falls to 105, a surplus of 150 minus 105 = 45 units, 30.0% of quantity supplied.
A minimum wage is the same diagram, except the surplus is labour, which reads as unemployment.
Under a floor, sellers compete in other ways: quality cuts, tied contracts, shorter probations. None of that appears in the diagram, and all of it earns marks.
A minimum wage is the same diagram, except the surplus is labour, which reads as unemployment.
Under a floor, sellers compete in other ways: quality cuts, tied contracts, shorter probations. None of that appears in the diagram, and all of it earns marks.
Ask students: what the surplus is called in a labour market (unemployment).
Scene 3: A quota makes the supply curve vertical
A quota of 100 units is 16.7% below the equilibrium 120: supply goes vertical at 100, and price climbs along demand to 58, 16% above equilibrium.
The tool differs from a ceiling: one caps price, the other caps quantity. A capped price creates a shortage of willing buyers; a quota guarantees that some buyers cannot get the good at all.
The blocked trades are the deadweight loss: 120 units were traded before, 20 fewer now, and both sides lose that surplus.
The tool differs from a ceiling: one caps price, the other caps quantity. A capped price creates a shortage of willing buyers; a quota guarantees that some buyers cannot get the good at all.
The blocked trades are the deadweight loss: 120 units were traded before, 20 fewer now, and both sides lose that surplus.
Ask students: how a quota differs from a ceiling on the diagram (a vertical line against a horizontal one).
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Calculation | Shortage or surplus | Place the control, then subtract the two quantities | 50 units |
| Calculation | Quota cut | (Equilibrium minus quota) divided by equilibrium | 16.7% |
| Explanation | Non-price competition | Shortage, then queues, ballots, black markets | At least two |
| Essay | Policy evaluation | Winners, losers, deadweight loss, alternatives | Write the trade-off |
Sanity check (teach this): A ceiling must sit below equilibrium and a floor above it; reversing the direction loses the whole item.
Shortage is quantity demanded minus supplied, surplus is supplied minus demanded, never the other way round.
Keep denominators consistent: 35.7% uses 140, while 30.0% uses 150.
A price change is a movement along the curve, not a shift; only non-price factors shift the curve.
Shortage is quantity demanded minus supplied, surplus is supplied minus demanded, never the other way round.
Keep denominators consistent: 35.7% uses 140, while 30.0% uses 150.
A price change is a movement along the curve, not a shift; only non-price factors shift the curve.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Shortage at a ceiling of 40 | 50 units | 140 minus 90 |
| Surplus at a floor of 60 | 45 units | 150 minus 105 |
| Cut from a quota of 100 | 16.7% | (120 minus 100) / 120 |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Drawing the control on the wrong side | A ceiling above equilibrium | A ceiling above equilibrium binds nothing, so the whole item scores zero. |
| Confusing demand with quantity demanded | Saying a ceiling raises demand | Quantity demanded rises along the curve; the curve itself does not move. |
| Swapping shortage and surplus | Saying a ceiling causes a surplus | A ceiling holds price down, so more want to buy: that is a shortage. |
| Stopping at the diagram | Writing “there is a shortage” and stopping | Add queues, black markets and deadweight loss. |
07 Homework (paper, section and question numbers)
- Secondary economics chapter index — All chapters and practice
- All lesson-prep guides — Compare other subjects
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Shortage at a ceiling of 40 | 50 units | All three right: move to tax and subsidy |
| Quota cut | 16.7% | One wrong: redraw the diagram |
| Surplus at a floor of 60 | 45 units | Two wrong: return to act two |
09 Tutor FAQ
Why does a ceiling above equilibrium do nothing?
The market price is already lower, so the law binds no trade.
Can a quota and a ceiling be combined?
Yes, but then the diagram carries both a vertical and a horizontal line; papers usually test them separately.
Must deadweight loss be computed?
At school level, naming the region and the reason is enough unless figures are given.
Find a tutor — freeTutor Registration →
A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
