IBonomics2026-W34
Joint-supply problems and shift diagrams drove most errors
Aggregate quiz data show large gaps in applying joint‑supply logic and simultaneous demand/supply shift diagrams — revise those topics first.
Published August 30, 2026
Total attempts
144 attempts
Active students
30 users
Average score
68.5%
Overall question accuracy
71.3%
Most-missed item correct rate
0% (joint-supply item)
1. Weekly Overview
Joint supply and shift-analysis were weakest this week.
Across 144 attempts from 30 learners, items involving joint supply (e.g., how production of one good affects a jointly produced good) and combined shift diagrams produced the lowest accuracy and longest thinking times on hard questions. Standard supply-shift causes (taxes, expectations) remain a clear strength.
Student takeaway: Start by revising joint‑supply logic (unit 2.2) and practice combined demand/supply shifts with clear diagrams.
2. Key Patterns
Main Insights
Joint-supply reasoning is the biggest weakness
Three of the top five most-missed items concern joint supply (e.g., leather vs beef, eggs vs chicken) with correct rates near 0–9% and many attempts, showing systematic misunderstanding of how joint production links supply curves.
Advice: Revise unit 2.2: practice identifying when goods are jointly supplied, then map how a demand or price change for one good shifts the other's market supply using step-by-step cause→effect diagrams.
Confusion about supply responses when demand shifts
Hard items that ask students to combine demand increases with producers' short-run responses (e.g., concert tickets) had very low correct rates and high wrong rates, indicating a common incorrect belief that higher demand automatically raises supply instantly.
Advice: Practice comparative statics: draw the initial equilibrium, shift only the demand curve, identify the new short-run price and quantity, and only then consider supply responses that require time or additional information.
Clear strength on standard supply-shift causes
Questions about predictable supply shifts — e.g., subsidy removal, taxes, and producers' expectations — had very high correct rates (many at 100%), showing good recall of common policy and expectation effects on supply curves.
Advice: Keep using short diagram practice for taxes, subsidies and expectations (unit 2.2) but focus revision time on applying those rules to joint‑supply and simultaneous shifts.
3. Revision Plan
Priority Areas
Joint supply (effects across jointly-produced goods) · 2.2
Multiple hard items on joint supply had near-zero correct rates and high attempts.
Action: Work through 6–8 targeted examples: identify joint outputs, state how a demand/price change for one good alters the other's supply, and draw separate supply diagrams for each market.
Combined demand & supply shifts (short-run vs longer-run responses) · 2.1 / 2.2
Students confused short-run comparative statics versus supply adjustments that require time or incentives.
Action: Practice single-diagram exercises where only one curve shifts first, then add secondary shifts; annotate steps and justify why supply would or would not move immediately.
Simultaneous shocks in one diagram (tax + income change) · 2.2
Questions asking for combined policy and income shocks (e.g., petrol tax + income fall) showed low accuracy.
Action: Do 4–5 mixed-shock diagram problems, state net effects on price and quantity, and list assumptions used for net-direction conclusions.
Numerical elasticity and PES calculations · 2.5 / 2.6
Time-spent outliers and low accuracy on elasticity/PES calculations indicate practice gaps.
Action: Practice midpoint elasticity and PES problems with stepwise calculation and interpret revenue/supply implications in 10–15 minute timed drills.
4. Question Evidence
Featured Questions
Most Wrong Questions
| Question | Topic | Unit | Attempts | Correct | Wrong | Avg Time |
|---|---|---|---|---|---|---|
| Demand for beef falls, leading to a lower price and reduced production of beef. Beef and leather are jointly supplied from the same cows. What happens to the market supply of leather? | Microeconomics | 2.2 | 31 | 0.0% | 100.0% | 13.2 s |
| A student says: “When demand for concert tickets increases, firms will increase their supply, so prices will fall back down.” Using a demand and supply diagram, what actually happens when demand for concert tickets rises? | Microeconomics | 2.1 | 33 | 6.1% | 93.9% | 23.8 s |
| Demand for chicken rises, increasing the price and production of chicken. Eggs and chicken meat are in joint supply. What happens to the market supply of eggs? | Microeconomics | 2.2 | 34 | 8.8% | 91.2% | 14.1 s |
| Suppose beef and pork are substitutes, while beef and barbecue sauce are complements. The price of pork rises and the price of barbecue sauce also rises. If the substitution effect from pork to beef is known to be stronger than the negative effect from the higher barbecue sauce price, what is the net effect on the demand for beef? | Microeconomics | 2.1 | 34 | 8.8% | 91.2% | 7.6 s |
| In a single diagram for petrol, show the impact of a new indirect carbon tax on petrol and a simultaneous fall in household incomes during a recession. | Microeconomics | 2.2 | 16 | 18.8% | 81.3% | 10.5 s |
Most Correct Questions
| Question | Topic | Unit | Attempts | Correct | Wrong | Avg Time |
|---|---|---|---|---|---|---|
| A government decides to cut or remove existing subsidies for public universities. What happens to the market supply of university places in the short run? | Microeconomics | 2.2 | 33 | 100.0% | 0.0% | 7.2 s |
| Oil producers expect the price of oil to rise sharply next month. What is likely to happen to the current market supply of oil? | Microeconomics | 2.2 | 31 | 100.0% | 0.0% | 6.5 s |
| If the price of printers decreases, what happens to the demand for printer ink (a complementary good)? | Microeconomics | 2.1 | 31 | 100.0% | 0.0% | 7.0 s |
| The government introduces a new indirect tax on hotel stay. What happens to the hotel industry's supply curve? | Microeconomics | 2.2 | 31 | 100.0% | 0.0% | 5.6 s |
| Global coffee prices stay high for several years after a demand surge; in the long run, what happens to the market supply of coffee? | Microeconomics | 2.2 | 33 | 93.9% | 6.1% | 16.0 s |
Most Time Spent Questions
| Question | Topic | Unit | Attempts | Correct | Wrong | Avg Time |
|---|---|---|---|---|---|---|
| US multinational banks scale back plans for new branches in South Africa and keep more investment at home. How does this fall in outward FDI affect the USD supply curve in the foreign exchange market? | Global Economy | 4.5 | 7 | 100.0% | 0.0% | 77.2 min |
| In IBonomica, the average monthly spending per person on wood logs is £76.53, which includes a 20% VAT applied at the point of sale. Using this information, calculate how much of the £76.53 is the actual value of the wood logs before VAT. | microeconomics | 2.7 | 4 | 50.0% | 50.0% | 36.1 min |
| The price of a pair of brand‑name headphones falls from $20 to $18, and weekly sales rise from 120 to 140 units. Calculate the price elasticity of demand (midpoint method) and state whether total revenue will rise or fall. | microeconomics | 2.5 | 4 | 25.0% | 75.0% | 5.4 min |
| Currency traders lose confidence in the US dollar and quickly move their funds into other currencies. What is the likely impact on the price of the dollar in the foreign exchange market? | Global Economy | 4.5 | 4 | 50.0% | 50.0% | 4.8 min |
| The price of laptops falls from $1,000 to $950. If the price elasticity of supply is 0.8 and producers originally supplied 20,000 units, calculate the new quantity supplied. | microeconomics | 2.6 | 4 | 25.0% | 75.0% | 3.7 min |
5. Conclusion
Next steps
Focus short sessions on joint‑supply problems and stepwise diagram practice for combined shifts. Use a few timed calculation drills for elasticity/PES to rebuild speed and accuracy.
Note: This weekly report is automatically generated and may contain mistakes. Always double-check key points before using them for revision.
This report is based on anonymized aggregate quiz activity and statistics from the indicated weekly summary; it does not include or reveal any individual student data.