Karnataka II PUC Economics (KSEAB) common mistakes
Karnataka II PUC Economics (KSEAB) · Understand the traps and how to avoid them.
Writing that a 'change in quantity demanded' and a 'change in demand' are the same thing, or labelling a price-induced movement along the curve as a shift of the demand curve.
The two phrases sound interchangeable in everyday English, and the textbook distinction (movement along the curve versus shift of the curve) is only enforced in diagram-based questions, so candidates memorise the definitions without attaching them to a diagram.
What to do instead
Before answering, ask 'did price change or did something else change?' If price changed, draw arrows along the existing curve and write 'movement along the demand curve - change in quantity demanded'. If income, tastes, price of related goods or expectations changed, draw a new parallel curve and write 'shift - change in demand'. Practise by drawing both cases for the same good side by side and labelling the axes every time.
Confusing the income effect and substitution effect, or claiming that both always push quantity demanded in the same direction.
Both effects are taught in the same lesson with the same diagram, and candidates memorise the labels without understanding that the substitution effect is always negative (price up, quantity down) while the income effect's direction depends on whether the good is normal or inferior.
What to do instead
Anchor each effect to its cause: substitution effect = relative price change with real income held constant; income effect = change in real purchasing power with relative prices held constant. Then state the rule explicitly for each good type: for a normal good both effects reinforce each other; for an inferior good they oppose each other; for a Giffen good the income effect dominates. Write one sentence per case in your own words until you can state the direction without a diagram.
Describing diminishing marginal utility as 'total utility falls' rather than 'total utility rises at a decreasing rate', and drawing the TU curve as downward sloping from the start.
The word 'diminishing' is read as 'decreasing', so candidates assume total utility must be falling. The distinction between 'falling' and 'rising more slowly' is a subtle one that is easy to gloss over when revising from a summary.
What to do instead
Learn the three-phase rule for total utility: rises at an increasing rate while marginal utility rises, rises at a decreasing rate while marginal utility is positive but falling, reaches maximum where marginal utility is zero, and falls only when marginal utility becomes negative. Draw the MU curve below the TU curve with the zero-MU point aligned exactly under the TU peak, and check that alignment in every practice diagram.
Mixing up the elasticity formulas - using the percentage change in price over the percentage change in quantity, or computing elasticity along a demand curve without the negative sign convention.
Candidates memorise 'change in quantity over change in price' and then invert the ratio under exam pressure, or forget that the formula uses percentage changes, not absolute changes, so the answer changes when units change.
What to do instead
Write the formula at the top of every numerical answer: Ed = (percentage change in quantity demanded) / (percentage change in price). Convert to percentages first, then divide. State the convention you are using: price elasticity of demand is usually reported as an absolute value, so note the negative sign and then take the modulus. Sanity-check the result: if the answer exceeds 1 for a good with many substitutes, that is plausible; if it exceeds 1 for salt, recheck your arithmetic.
Confusing the law of variable proportions with returns to scale, or drawing the short-run average cost curve as U-shaped without explaining why it is U-shaped.
Both topics involve 'increasing, constant and decreasing returns' and are revised close together, so the labels blur. The U-shape of AC is often memorised as a fact rather than derived from the interaction of AFC and AVC.
What to do instead
Separate the two by the question 'is at least one factor fixed?' If yes, it is the short run and you are discussing the law of variable proportions (returns to a variable factor). If all factors change proportionately, it is returns to scale. For the U-shape, draw AFC falling continuously, AVC falling then rising, and show that AC = AFC + AVC, so AC falls while the fall in AFC outweighs the rise in AVC and rises once the rise in AVC dominates. Reproduce this three-curve diagram from memory at least twice.
Stating that a firm shuts down when price falls below average total cost, instead of below average variable cost.
The average total cost curve is the one drawn most prominently in diagrams, and the shutdown point on the AVC curve is easy to overlook. Candidates also conflate the shutdown point with the break-even point.
What to do instead
Fix the two thresholds with a one-line test. Break-even: price = minimum ATC, the firm earns normal profit and stays in the short run. Shutdown: price = minimum AVC, the firm covers variable costs but not fixed costs; below this point, producing adds to losses, so output falls to zero. In the short run fixed costs are sunk, so they do not affect the shutdown decision. Draw the MC, AVC and ATC curves and mark both points on the MC curve.
Treating the supply curve as the firm's marginal cost curve without noting that the firm's short-run supply curve is only the portion of MC above minimum AVC.
The statement 'supply curve equals MC' is repeated in summaries, and the qualifying clause about the portion above AVC is dropped because it seems like a technicality.
What to do instead
Write the full statement each time: the short-run supply curve of a competitive firm is the rising part of its marginal cost curve from the point where MC cuts AVC at its minimum upward. Explain in one sentence why the segment below minimum AVC is excluded: at those prices the firm produces nothing rather than producing at a loss greater than its fixed costs.
Describing perfect competition as 'many buyers and sellers selling similar goods' and then using that same description for monopolistic competition, so the two markets become indistinguishable in the answer.
Both markets feature many sellers, so candidates list the overlapping features and stop, missing the features that actually separate them: product homogeneity versus product differentiation, and price taker versus downward-sloping demand.
What to do instead
Build a comparison table with these rows: number of sellers, nature of the product, freedom of entry and exit, shape of the firm's demand curve, price-setting power, and long-run profit. Fill it for all four market forms. The decisive rows are product homogeneity (perfect competition) versus differentiation (monopolistic competition), and horizontal demand (price taker) versus downward-sloping demand (price maker). In any 'distinguish' question, lead with the decisive difference before listing secondary ones.
Claiming a profit-maximising firm produces where price equals marginal cost, without checking the second-order condition or distinguishing it from where MR equals MC.
The MR = MC rule and the P = MC rule are both taught, and candidates apply the wrong one to the wrong market structure. Under perfect competition P = MR, so the two rules coincide and the error stays hidden until a monopoly question appears.
What to do instead
State the general rule first: any firm maximises profit where MR = MC and MC is rising. Then specialise: under perfect competition P = MR, so the rule becomes P = MC; under monopoly P > MR, so the firm sets output where MR = MC and reads price off the demand curve above that output. Always check that MC cuts MR from below, since an intersection with falling MC is a profit minimum.
Writing that equilibrium price is determined by demand alone or by supply alone, or shifting only one curve when the question states that both demand and supply change.
Most textbook diagrams show one curve shifting at a time, so candidates default to a single shift even when the question describes two simultaneous changes. The effect on price and quantity then depends on which shift is larger, which requires an explicit comparison.
What to do instead
For any equilibrium question, list every change mentioned and match it to a curve before drawing. If both curves shift, draw the sequence and state the result in two parts: the direction of the price change and the direction of the quantity change, noting which is certain and which is ambiguous. For example, if demand and supply both rise, quantity certainly rises but the price change depends on the relative size of the shifts. Write that caveat explicitly to earn the reasoning marks.