CLEP Principles of Microeconomics Quick Review

Scarcity forces every economic actor to choose, and the production possibilities curve (PPC) is the single graph that captures scarcity, opportunity cost, efficiency, and growth at once.

Opportunity cost of a decision = the value of the single highest-valued alternative forgone, not the sum of everything given up. On a PPC: points ON the curve are productively efficient; points INSIDE are attainable but inefficient (unemployed/underused resources); points OUTSIDE are currently unattainable. A bowed-out (concave) PPC reflects increasing opportunity cost (resources are imperfect substitutes between the two goods); a straight-line PPC reflects constant opportunity cost. More/better resources or new technology shift the whole curve outward (economic growth).

Opportunity cost (forward-looking, belongs in the decision) vs.\ sunk cost (already spent, cannot be recovered, should be ignored when deciding what to do next).

Students label every point off the curve "impossible," forgetting that points inside the curve are attainable, just inefficient --- only points outside are currently unattainable given current resources and technology.

On curve = efficient; inside = unemployed; outside = impossible (for now).

Trade creates mutual gains when each party specializes in the good for which it has the lowest opportunity cost, even if one party is better at producing everything.

Absolute advantage = can produce MORE output using the same resources (compare output levels or resource requirements). Comparative advantage = LOWER opportunity cost of producing a good (compare opportunity costs, not output). Mutually beneficial terms of trade must fall between the two producers' opportunity-cost ratios (autarky price ratios) for the traded good.

Absolute advantage (who produces more, or uses fewer resources) vs.\ comparative advantage (who gives up less) --- a country can hold the absolute advantage in both goods yet the comparative advantage in only one.

Students assign the "advantage to trade" to whoever has higher output or a lower total resource cost, when only opportunity cost (comparative advantage) determines who should specialize in what.

Absolute = more output; Comparative = lower opportunity cost $arrow$ trade on comparative.

Market economies coordinate scarce resources through decentralized price signals rather than central planning, and economists carefully separate testable facts from value judgments.

Every economy must answer: What to produce? How to produce it? For whom to produce it? Market, command, and mixed economic systems answer these three questions differently, largely based on who owns productive resources. The circular-flow diagram shows households and firms exchanging resources and goods through resource markets and product markets, with money flowing opposite to the direction of goods/resources. Positive statements are fact-based and testable ("what is"); normative statements are opinion-based ("what should be").

Positive economics (provable, fact-based claims) vs.\ normative economics (opinion, "should"/"better"/"fair" claims) --- a statement can contain a factual number and still be normative if it also passes a value judgment.

Students see a factual number inside a sentence and call it "positive" even though the sentence also contains a value-judgment word like "should" or "fair," which makes the whole statement normative.

Positive = provable fact; normative = opinion/should.

Rational decision-makers compare marginal benefit to marginal cost and act only as long as the marginal benefit of one more unit exceeds its marginal cost.

Optimal quantity occurs where $MB = MC$; if $MB>MC$, do more; if $MB<MC$, do less. This marginal-analysis rule underlies nearly every optimization decision tested on this exam: consumer choice, firm output, and resource hiring all reduce to a marginal-benefit-equals-marginal-cost condition.

Marginal values (the change from one more unit) vs.\ average or total values (whole-quantity measures) --- a rational actor decides at the margin, not by comparing totals or averages.

Students try to maximize total benefit or minimize total cost instead of stopping at the quantity where marginal benefit equals marginal cost, causing over- or under-production/consumption in the answer choice they select.

Do it while $MB > MC$; stop exactly where $MB = MC$.

The law of demand says price and quantity demanded move in opposite directions along one fixed demand curve, while non-price determinants shift the entire curve.

Demand shifters: Tastes/preferences, Related goods' prices (substitutes/complements), Income (normal vs.\ inferior goods), number of Buyers, Expectations of future price. A rise in income increases demand for a normal good but decreases demand for an inferior good.

A "change in demand" (whole curve shifts, from a non-price determinant) vs.\ a "change in quantity demanded" (movement along a fixed curve, from a change in that good's own price).

Students shift the entire demand curve in response to a price change, or slide along the existing curve when a non-price determinant (income, tastes) actually changed.

Price change $arrow$ move along the curve; determinant change $arrow$ shift the curve.

The law of supply says price and quantity supplied move in the same direction along a fixed supply curve, while non-price determinants shift the entire curve.

Supply shifters: Resource/input prices, prices of other goods a firm could produce, Technology, Taxes and subsidies, Expectations, number of Sellers. A per-unit tax on producers shifts supply left (up); a subsidy shifts supply right (down).

A "change in supply" (shift from a determinant) vs.\ a "change in quantity supplied" (movement along the curve from a price change) --- the same distinction as on the demand side.

Students treat a rise in input prices as directly raising market price, when it actually shifts supply LEFT first --- the new equilibrium price is a downstream consequence, not the shift itself.

Input costs up $arrow$ supply shifts LEFT (less supplied at every price).

Equilibrium price and quantity occur where supply and demand intersect --- the only point where quantity supplied equals quantity demanded and price feels no pressure to change.

Price above equilibrium: $Q_s>Q_d$ $arrow$ surplus, pushes price down. Price below equilibrium: $Q_d>Q_s$ $arrow$ shortage, pushes price up. When supply and demand shift simultaneously, one of price or quantity is determinate and the other is ambiguous without knowing the relative size of the two shifts.

Surplus (excess supply, price too high, sits ABOVE equilibrium price) vs.\ shortage (excess demand, price too low, sits BELOW equilibrium price).

On simultaneous double-shift questions, students force a definite answer for both price AND quantity when only one is actually determinate without knowing which shift is larger.

Price too high $arrow$ surplus; price too low $arrow$ shortage.

Price elasticity of demand measures how responsive quantity demanded is to a price change, and its value tells you exactly how total revenue will move.

$E_d = ≤ft|/% Q_d% P|$, computed with the midpoint (arc) method: $/(Q_2-Q_1)/[(Q_1+Q_2)/2](P_2-P_1)/[(P_1+P_2)/2]$. $E_d>1$: elastic; $E_d<1$: inelastic; $E_d=1$: unit elastic. Determinants: availability of substitutes, necessity vs.\ luxury, share of budget, time horizon --- more elastic with more substitutes, more luxury-like, larger budget share, longer time to adjust.

Elastic demand (a small % price change causes a LARGER % change in quantity) vs.\ the numeric slope of the demand line, which is NOT the same thing as elasticity.

Students use the plain %-change formula (dividing by the original value only) instead of the midpoint method, so the elasticity value changes depending on the direction of the price change --- the exam expects the midpoint method whenever two full price/quantity pairs are given.

$E_d>1$ elastic, $E_d<1$ inelastic, $E_d=1$ unit elastic --- always midpoint method.

The total-revenue test links elasticity directly to what happens to seller revenue when price changes, and analogous formulas measure supply, cross-price, and income responsiveness.

Total revenue $TR = P × Q$. Elastic demand: price and $TR$ move in OPPOSITE directions. Inelastic demand: price and $TR$ move in the SAME direction. Unit elastic: $TR$ is at its maximum. Cross-price elasticity $E_xy = % Q_x / % P_y$: positive $arrow$ substitutes, negative $arrow$ complements. Income elasticity $E_i = % Q_d/% Income$: positive $arrow$ normal good, negative $arrow$ inferior good. Price elasticity of supply $E_s = % Q_s/% P$, generally more elastic over longer time horizons.

Cross-price elasticity's SIGN (positive/negative reveals substitute vs.\ complement) vs.\ price elasticity of demand, whose sign is conventionally dropped by taking an absolute value.

Students assume raising price always raises revenue; on the ELASTIC portion of demand, a price increase actually LOWERS total revenue because the % drop in quantity outweighs the % rise in price.

Elastic $arrow$ price & $TR$ move opposite; inelastic $arrow$ price & $TR$ move together.

Consumer and producer surplus measure the net benefit buyers and sellers get from trading at the market price, and their sum is maximized at the competitive equilibrium.

Consumer surplus = the area below demand and above the market price. Producer surplus = the area above supply and below the market price. Total (social) surplus $=CS+PS$, maximized at the free-market equilibrium quantity where marginal benefit equals marginal cost --- the definition of allocative efficiency.

Consumer surplus (a region ABOVE price, BELOW demand) vs.\ producer surplus (a region BELOW price, ABOVE supply) --- these two are routinely swapped.

When a price control or tax restricts quantity below equilibrium, students recompute $CS$ and $PS$ at the new price but forget the deadweight-loss triangle representing the surplus lost from trades that no longer happen.

CS = below demand, above price; PS = above supply, below price.

Binding price controls set below or above equilibrium prevent the market from clearing, creating a persistent shortage or surplus plus a deadweight loss.

A price ceiling set BELOW equilibrium is binding and causes a shortage ($Q_d>Q_s$). A price floor set ABOVE equilibrium is binding and causes a surplus ($Q_s>Q_d$). A control on the "wrong side" of equilibrium is non-binding and has no market effect. Both binding controls create deadweight loss because the quantity actually traded is restricted below the efficient (equilibrium) quantity.

A price ceiling (maximum legal price, e.g.\ rent control) vs.\ a price floor (minimum legal price, e.g.\ minimum wage) --- a ceiling caps price from above, a floor supports price from below.

Students assume the quantity actually traded under a shortage equals quantity demanded at the controlled price; it is really the SHORT side of the market ($Q_s$ under a binding ceiling, $Q_d$ under a binding floor).

Ceiling below eq.\ $arrow$ shortage; floor above eq.\ $arrow$ surplus; $Q$ traded = the SHORT side.

A per-unit tax drives a wedge between the price buyers pay and the price sellers receive, and the more inelastic side of the market bears the larger share of that burden.

An excise tax $t$ shifts supply up (or demand down) by $t$, creating a vertical tax wedge between $P_buyer$ and $P_seller$ at the new (reduced) quantity; tax revenue $= t × Q_new$. Tax incidence: whichever side --- buyers or sellers --- is relatively more INELASTIC bears the larger share of the burden. A subsidy works in reverse: it shifts supply down/right and creates an analogous wedge and deadweight loss.

Tax revenue (the wedge RECTANGLE, $t× Q_new$) vs.\ deadweight loss (the TRIANGLE of surplus lost from trades that no longer occur because $Q$ fell from $Q_eq$ to $Q_new$).

Students assume a tax legally imposed on sellers is automatically paid entirely by sellers (or vice versa) --- statutory incidence is irrelevant; economic incidence depends only on relative elasticities.

More inelastic side pays more of the tax; revenue = rectangle, DWL = triangle.

Opening a market to trade at a world price below the domestic equilibrium benefits consumers and hurts domestic producers, and tariffs/quotas claw back some of those free-trade gains at the cost of overall efficiency.

If world price $P_w<$ domestic equilibrium price, the country imports; domestic $Q_d$ rises, domestic $Q_s$ falls, imports $=Q_d-Q_s$ at $P_w$. A tariff raises the domestic price toward the domestic equilibrium, decreasing imports, raising producer surplus, lowering consumer surplus, generating government revenue, and creating two deadweight-loss triangles. A quota restricts import quantity directly, with similar welfare effects but no automatic government revenue.

A tariff (a tax on imports that generates government revenue) vs.\ a quota (a quantity limit on imports, no direct government revenue unless auctioned).

Students assume free trade must help everyone; in fact, with $P_w$ below the domestic price, DOMESTIC PRODUCERS lose surplus even though consumers and total surplus gain overall.

$P_w<P_domestic⇒$ import; consumer gain $>$ producer loss $⇒$ net national gain.

Consumers derive satisfaction (utility) from consumption, but the law of diminishing marginal utility explains why demand curves slope downward.

Total utility ($TU$) is the overall satisfaction from all units consumed; marginal utility $MU = / TU Q$ is the extra satisfaction from one more unit. Law of diminishing marginal utility: as a consumer consumes more units of a good in a given period, each additional unit adds LESS satisfaction than the previous one. $TU$ is maximized where $MU = 0$; $TU$ falls once $MU$ turns negative.

Diminishing marginal utility (each extra unit adds less, but total utility can still rise) vs.\ negative marginal utility (an extra unit actually lowers total utility, so $TU$ is falling).

Students read a falling $MU$ value as proof that $TU$ itself is falling; $TU$ keeps rising as long as $MU$ stays positive, even while $MU$ is declining.

$MU$ falling but positive $arrow$ $TU$ still rising; $MU<0 arrow$ $TU$ falling.

A utility-maximizing consumer allocates a limited budget across goods so that the last dollar spent on each good yields the same marginal utility.

Utility-maximizing rule: $/MU_xP_x = /MU_yP_y$ (equal marginal utility per dollar across all goods purchased), subject to the budget constraint $P_xQ_x + P_yQ_y = Income$. If $/MU_xP_x > /MU_yP_y$, the consumer gains total utility by buying more $X$ and less $Y$ until the ratios are equalized.

Comparing raw marginal utilities ($MU_x$ vs.\ $MU_y$) vs.\ comparing marginal utility PER DOLLAR ($MU_x/P_x$ vs.\ $MU_y/P_y$) --- only the per-dollar comparison determines the optimal spending shift.

Given two goods' prices and marginal utilities, students shift spending toward the good with the higher marginal utility instead of the higher marginal-utility-per-dollar, misapplying the equal-marginal-utility-per-dollar rule.

Maximize utility where $MU_x/P_x = MU_y/P_y$, spending the whole budget.

In the short run, at least one input is fixed, and adding more of the variable input eventually runs into diminishing marginal returns.

Marginal product $MP_L = / TP L$; average product $AP_L = /TPL$. Law of diminishing marginal returns: as more of a variable input (labor) is added to a fixed input (capital), $MP_L$ eventually declines. $MP$ crosses $AP$ exactly at $AP$'s maximum --- when $MP>AP$, $AP$ rises; when $MP<AP$, $AP$ falls.

Diminishing marginal returns (marginal product still positive but SMALLER) vs.\ negative marginal returns (an additional worker actually REDUCES total product) --- total product keeps rising in the first case, falls in the second.

Students assume diminishing marginal returns means total product is falling; $TP$ actually keeps rising, just at a decreasing rate, until $MP$ turns negative.

$MP$ crosses $AP$ at $AP$'s peak; $MP<0⇒ TP$ falling.

Because $MP$ and cost are mirror images of each other, the shape of the short-run cost curves is dictated entirely by the production function.

$TC=TFC+TVC$; $AFC=TFC/Q$ (always falling); $AVC=TVC/Q$; $ATC=TC/Q=AFC+AVC$; $MC=/ TC Q=/ TVC Q$. $MC$ intersects both $AVC$ and $ATC$ at each curve's OWN minimum point. $MC$ falls while $MP_L$ rises and rises once $MP_L$ falls --- cost curves mirror product curves.

Average variable cost ($AVC$, U-shaped, eventually rises) vs.\ average fixed cost ($AFC$, continuously falls toward zero, never rises).

Students draw $MC$ crossing $ATC$ and $AVC$ at the SAME quantity; $MC$ actually crosses $AVC$'s minimum at a LOWER $Q$ than where it crosses $ATC$'s minimum, since $AVC<ATC$ everywhere.

$MC$ cuts $AVC$ and $ATC$ at each curve's OWN minimum, hitting $AVC$'s min first.

In the long run all inputs are variable, so the long-run average total cost (LRATC) curve's shape reflects returns to scale rather than diminishing marginal returns.

Economies of scale: $LRATC$ falls as $Q$ rises (specialization, bulk buying, spreading fixed costs). Diseconomies of scale: $LRATC$ rises as $Q$ rises (coordination and communication costs). Constant returns to scale: $LRATC$ is flat. Minimum efficient scale (MES) is the lowest quantity at which $LRATC$ reaches its minimum.

Economies of SCALE (long-run, all inputs variable, shown by $LRATC$'s shape) vs.\ diminishing marginal returns (strictly SHORT-run, at least one fixed input) --- never use "diminishing returns" to explain a rising $LRATC$.

Students explain a rising $LRATC$ using "diminishing marginal returns," a short-run-only concept; the correct long-run explanation is diseconomies of scale.

$LRATC$ falling = economies of scale; rising = diseconomies; flat = constant returns.

Economists count implicit (opportunity) costs that accountants ignore, so a firm can show "normal" accounting profit while earning exactly zero economic profit.

Accounting profit $=TR-explicit costs$. Economic profit $=TR-(explicit+implicit costs)=$ Accounting profit $-$ implicit costs. Normal profit = the accounting-profit level at which economic profit $=0$ --- the firm covers the full opportunity cost of the owner's resources, with no incentive to enter or exit the industry.

Zero economic profit (the firm is FINE, covering all opportunity costs) vs.\ zero accounting profit (the firm is actually failing to cover its explicit costs).

Students treat "zero economic profit" as failure and assume the firm should exit; zero economic profit in the long run is the NORMAL, stable outcome for perfectly competitive firms.

Economic profit $=0⇒$ normal, sustainable --- not a failing firm.

Every firm, in every market structure, maximizes profit (or minimizes loss) by producing the quantity where marginal revenue equals marginal cost, and then checks whether it should even be operating.

Profit-maximizing rule: produce where $MR=MC$. If $MR>MC$, output should increase; if $MR<MC$, output should decrease. Total profit at that quantity $=(P-ATC)× Q$. Short-run rule: produce (even at a loss) if $P≥q AVC$; shut down immediately if $P<AVC$. Long-run rule: stay in the industry if $P≥q ATC$; exit if $P<ATC$.

Shutting down (temporary, short run --- fixed costs still paid, firm can reopen) vs.\ exiting (permanent, long run --- firm avoids ALL costs, including fixed, by leaving the industry).

Students shut down a firm the moment it shows an accounting loss ($P<ATC$) in the short run; the correct short-run test is $P$ vs.\ $AVC$ --- keep operating at a loss as long as variable costs are covered.

Always $MR=MC$ to find $Q^*$; short run shut down if $P<AVC$, long run exit if $P<ATC$.

In perfect competition, many small price-taking firms selling an identical product drive long-run economic profit to exactly zero at the lowest-cost, most efficient scale.

Characteristics: many buyers/sellers, homogeneous product, free entry/exit, perfect information. The price-taking firm's demand curve is perfectly horizontal at market price: $P=MR=AR=D$. Long-run equilibrium: entry/exit continues until $P=MC=ATC_min$ --- zero economic profit, productive efficiency (minimum $ATC$), and allocative efficiency ($P=MC$).

The firm's horizontal (perfectly elastic) demand curve vs.\ the market's downward-sloping demand curve --- the firm is tiny relative to the market and cannot affect price, but the market curve still slopes down.

Students draw the short-run profit graph correctly but forget the long-run adjustment: economic profit attracts entry, which shifts market supply right, driving $P$ down to $ATC_min$ and profit to zero.

Long run in perfect competition: $P=MC=ATC_min$, economic profit $=0$.

A monopolist is the sole seller facing the entire downward-sloping market demand curve, so selling more requires lowering price on ALL units, making marginal revenue fall below price.

For a monopolist, $MR<P$ at every quantity past the first unit, and (for linear demand) $MR$ bisects the horizontal distance between the vertical axis and demand --- twice as steep as demand. Profit-maximizing rule is still $MR=MC$; PRICE is then read off the DEMAND curve (never $MC$) at that quantity. Barriers to entry (economies of scale, control of a key resource, patents/licenses, network effects) let the monopoly persist.

Setting $P=MC$ (the perfectly competitive rule) vs.\ setting $MR=MC$ then pricing off demand (the monopoly rule) --- a monopolist never voluntarily prices where $P=MC$.

Students find the correct profit-maximizing quantity at $MR=MC$ but then read price off the $MC$ or $MR$ curve instead of going straight up to DEMAND --- the single most common monopoly-graphing error.

Find $Q$ at $MR=MC$; find $P$ by going UP to demand, never to $MC$.

Because a monopolist restricts output below the competitive (allocatively efficient) level to raise price, monopoly creates deadweight loss --- unless the firm can price-discriminate.

Monopoly deadweight loss = the triangle between $Q_monopoly$ and $Q_competitive$ (where demand crosses $MC$). Price discrimination requires market power, the ability to separate customers by willingness to pay, and no resale between groups. Perfect (first-degree) price discrimination charges each buyer their exact maximum willingness to pay: the firm captures ALL consumer surplus as producer surplus, produces the ALLOCATIVELY EFFICIENT quantity (where $D=MC$), and creates ZERO deadweight loss.

Price discrimination (different prices for the IDENTICAL good, based on the buyer) vs.\ product differentiation (different VERSIONS of a product sold at different prices).

Students assume price discrimination is always less efficient than single-price monopoly; PERFECT price discrimination is actually allocatively efficient (produces the competitive quantity), even though it eliminates consumer surplus entirely.

Perfect price discrimination: $Q=Q_competitive$, zero DWL, but ALL surplus goes to the firm.

Monopolistic competition blends monopoly's downward-sloping demand (from product differentiation) with perfect competition's free entry/exit, producing zero long-run economic profit at an inefficient scale.

Characteristics: many firms, differentiated products, some price-making power, free entry/exit. Short run: firm sets $MR=MC$, prices off its own downward-sloping demand curve, and can earn profit or loss. Long run: entry/exit shifts each firm's demand curve until it is tangent to $ATC$ --- $P=ATC$ (zero economic profit) but at a quantity LESS than $ATC_min$, creating "excess capacity."

Monopolistic competition's long-run zero profit (at $P=ATC$, but ABOVE $ATC_min$, allocatively inefficient) vs.\ perfect competition's long-run zero profit (at $P=ATC=ATC_min$, fully efficient).

Students conclude "zero economic profit" means the outcome is just as efficient as perfect competition; monopolistic competition's zero-profit point still has $P>MC$ --- allocatively inefficient.

Monopolistic competition long run: $P=ATC$ but NOT at $ATC_min$ --- excess capacity remains.

Oligopoly firms are mutually interdependent --- each must weigh rivals' likely reactions before changing price --- and the kinked demand curve model explains why oligopoly prices tend to be "sticky."

Oligopoly: few large firms, significant barriers to entry, homogeneous or differentiated product, strategic interdependence. Kinked demand curve: demand is relatively ELASTIC above the current price (rivals won't match a hike) and relatively INELASTIC below it (rivals WILL match a cut), creating a discontinuous gap in $MR$ at the kink --- $MC$ can shift within that gap without changing the profit-maximizing price.

Collusive (cartel) oligopoly behavior, where firms jointly act like a monopoly (illegal in the U.S.\ under antitrust law), vs.\ independent kinked-demand behavior, where firms avoid price wars without any agreement.

Students assume oligopoly prices respond readily to small cost changes as in perfect competition; the kinked-demand $MR$ gap is exactly why prices can stay sticky through a moderate $MC$ shift.

Elastic above the kink, inelastic below it $arrow$ gap in $MR$ $arrow$ sticky price.

Game theory models strategic interdependence among oligopolists with a payoff matrix, revealing why mutually beneficial collusion is hard to sustain even when it raises joint profit.

A dominant strategy is a choice that is best for a player regardless of the rival's choice. A Nash equilibrium is a strategy pair where neither player can improve their own payoff by unilaterally switching, given the other's choice. The Prisoner's Dilemma shows both firms have a dominant strategy to compete/cheat, landing at a Nash equilibrium with LOWER joint profit than mutual cooperation (collusion) --- why cartels tend to break down.

A dominant strategy (best regardless of the opponent's move --- doesn't always exist) vs.\ a Nash equilibrium (a mutual best-response outcome --- found by checking BOTH players' best responses).

Students check only one player's best responses when solving for the Nash equilibrium; a true Nash cell must be a mutual best response for BOTH firms simultaneously.

Nash: neither player wants to switch, given the other's choice --- check BOTH players.

A firm's demand for a factor of production (labor, capital) is "derived" from demand for the output that factor helps produce, and the firm hires up to where the added revenue from one more input unit equals its added cost.

Marginal Revenue Product $MRP = MP × MR$. If the output market is perfectly competitive, $MR=P$, so $MRP=MP× P$. Marginal Resource (Factor) Cost $MRC$ = the extra cost of hiring one more input unit. Profit-maximizing hiring rule: hire up to where $MRP=MRC$. Factor demand ($MRP$) shifts right when output price rises, the factor becomes more productive, a complementary resource's price falls, or a substitute resource's price rises.

$MRP$ (a DEMAND-side value --- what the firm gains from one more input unit) vs.\ $MRC$ (a SUPPLY-side cost --- what the firm pays for it) --- hiring sets these equal, just like $MR=MC$ in the output market.

Students compute $MRP=MP× P$ even when the firm's output market is imperfectly competitive; there, $MR<P$, so the correct formula is $MRP=MP× MR$.

Hire factor units until $MRP=MRC$; $MRP=MP× MR$ (or $MP× P$ only if output market is competitive).

In a perfectly competitive labor market, both firms and workers are "wage takers," so market-level supply and demand set the wage, and each firm simply hires at that given wage.

An individual firm hiring in a perfectly competitive resource market faces a perfectly ELASTIC (horizontal) labor supply curve at the market wage, so $MRC=$ wage $=$ the firm's supply curve --- the firm hires up to $MRP=MRC=$ wage. The MARKET labor supply (upward-sloping, reflecting the rising opportunity cost of an alternative use such as leisure) and market labor demand (downward-sloping) intersect to set that equilibrium wage.

The firm's horizontal resource supply curve in a competitive factor market vs.\ the market's upward-sloping resource supply curve --- the same firm/market distinction as in product markets, applied to inputs.

Students graph an individual competitive firm's hiring decision with an upward-sloping labor supply curve (that's the MARKET curve or a monopsonist's curve), not the horizontal curve a competitive firm actually faces.

Competitive firm in factor market: horizontal supply $=$ wage $=MRC$; hire where $MRP=$ wage.

A monopsonist is the sole (or dominant) buyer of a resource, so it faces the upward-sloping market resource supply curve directly --- mirroring monopoly on the selling side --- and ends up hiring fewer workers at a lower wage than a competitive market would.

A monopsonist's $MRC$ curve lies ABOVE its upward-sloping labor supply curve, because hiring one more worker requires raising the wage paid to ALL workers, not just the marginal hire. The hiring rule is still $MRP=MRC$, but the WAGE actually paid is read off the SUPPLY curve at that quantity --- producing a lower wage and lower employment than the competitive outcome.

Monopsony's underpayment of labor (wage $<MRP$, from restricting hiring) vs.\ monopoly's overcharging for output (price $>MC$, from restricting output) --- mirror-image market-power distortions.

Students find the monopsonist's hiring quantity correctly at $MRP=MRC$ but then read the wage off the $MRC$ curve instead of the labor SUPPLY curve at that quantity.

Monopsony: find $Q$ at $MRP=MRC$; find wage by dropping DOWN to labor SUPPLY.

An externality is an uncompensated spillover cost or benefit imposed on a third party outside the transaction, causing the free market to produce the "wrong" quantity relative to what's socially efficient.

Negative externality (e.g., pollution): marginal social cost $MSC>$ marginal private cost $MPC$; the unregulated market OVERPRODUCES relative to the efficient quantity. A per-unit Pigouvian tax equal to the externality shifts private supply up to $MSC$, correcting the overproduction. Positive externality (e.g., education): marginal social benefit $MSB>$ marginal private benefit $MPB$; the market UNDERPRODUCES; a per-unit subsidy corrects the underproduction.

Negative externality (social COST curve lies ABOVE private supply, $MSC>MPC$) vs.\ positive externality (social BENEFIT curve lies above private demand, $MSB>MPB$).

Students draw the externality correction as one new curve replacing the old one instead of showing both the private curve and the social curve together, with two distinct equilibrium points (market vs.\ socially efficient).

Negative externality: $MSC>MPC$, overproduced, tax it. Positive: $MSB>MPB$, underproduced, subsidize it.

Goods are classified by excludability (can non-payers be kept out?) and rivalry (does one person's use reduce what's left for others?), and this classification predicts whether private markets provide the efficient quantity.

Private goods: excludable AND rivalrous --- markets provide efficiently. Public goods: non-excludable AND non-rivalrous (e.g., national defense) --- the free-rider problem means private markets underprovide, so government typically provides them via taxes. Common resources: rivalrous but non-excludable --- subject to the "tragedy of the commons." Club goods: excludable but non-rivalrous (e.g., cable TV).

Non-excludable (can't stop free riders) vs.\ non-rivalrous (one person's use doesn't diminish another's) --- a good needs BOTH properties to be a true public good.

Students label any government-provided good a "public good" by definition; classification depends only on excludability/rivalry, not on who provides it (some public goods are privately funded; some excludable goods, like toll roads, are government-provided).

Public good $=$ non-excludable $+$ non-rivalrous $arrow$ free-rider problem $arrow$ government provides.

The tools government uses to correct inefficiency from market power --- price regulation, antitrust enforcement --- must be matched to the specific market structure and type of failure at hand.

Natural monopoly (economies of scale so large that one firm can serve the market at lowest cost) can be regulated at $P=MC$ (socially optimal, allocatively efficient, but often forces a loss needing a subsidy since $P<ATC$) or $P=ATC$ (fair-return, zero economic profit and sustainable, but not fully efficient). Antitrust laws prohibit anticompetitive practices such as price-fixing and market division among oligopolists and can block mergers that would substantially reduce competition.

Socially optimal ($P=MC$) regulation --- efficient but often causes the firm a loss --- vs.\ fair-return ($P=ATC$) regulation --- sustainable, zero profit, but NOT fully efficient.

Students assume any price regulation on a monopoly is automatically "the efficient outcome"; only $P=MC$ regulation is allocatively efficient, and it typically requires a government subsidy to keep the firm from operating at a loss.

Socially optimal: $P=MC$, efficient but may need a subsidy. Fair-return: $P=ATC$, sustainable but not efficient.

The Lorenz curve and the Gini coefficient derived from it are the standard tools for visualizing and quantifying how unequally income (or wealth) is distributed across a population.

The Lorenz curve plots cumulative % of income (vertical axis) against cumulative % of population, ordered poorest to richest (horizontal axis). The 45-degree diagonal represents PERFECT equality. Gini coefficient $=/area between the equality line and the Lorenz curvetotal area under the equality line$, ranging from 0 (perfect equality) to 1 (perfect inequality) --- the farther the Lorenz curve bows from the diagonal, the higher the Gini coefficient.

A Lorenz curve bowing FARTHER from the diagonal (MORE inequality, HIGHER Gini) vs.\ one moving CLOSER to the diagonal (LESS inequality, LOWER Gini) --- these directions are often reversed by mistake.

Students think the Gini coefficient can exceed 1 or go negative; by construction, with cumulative shares ordered poorest-to-richest, the Lorenz curve never crosses above the diagonal, so Gini stays bounded within $[0,1]$.

Lorenz curve farther from the 45$^$ line $=$ more inequality $=$ higher Gini (0 to 1).

Every optimization rule in the course reduces to marginal benefit $=$ marginal cost.

When two terms sound alike, ask which curve moves and which one only relabels a point.

Fewer firms & higher barriers $arrow$ more long-run price-making power.

Memorize each list as a block --- exam distractors swap one shifter for a look-alike from the wrong list.

Structure $arrow$ rule $arrow$ $Q^*$ $arrow$ correct curve for $P^*$ $arrow$ check against the rules, not intuition.

$≈$80 MCQ in 90 minutes, one sitting --- Product Markets alone is worth more than half the exam.

Firms $+$ product homogeneity $+$ entry barriers $=$ the three questions that pin down market structure.

Midpoint %$ Q$ over midpoint %$ P$, absolute value, then read $TR$ off the elasticity classification.