Consumer Theory: Utility, Demand & Equilibrium (ICs, Budget Lines, Elasticity)

TAKEAWAYS:

• Utility ≠ happiness: Cardinal measures utility in numbers (e.g., 10 utils for a burger), while ordinal ranks preferences (e.g., "I prefer A > B > C")—Nepal’s NEPSE stock rankings use ordinal utility.
• Budget lines show trade-offs: If your monthly salary is Rs. 30,000 and a smartphone costs Rs. 20,000, you can buy 1.5 smartphones (or 1 smartphone + Rs. 10,000 worth of other goods).
• Indifference curves are downward-sloping and convex: A consumer is equally happy with 2 kg of rice or 1 kg of rice + 1 kg of lentils (real example: NTC’s bundled electricity plans).
• Price elasticity of demand (PED) determines revenue: If Daraz lowers the price of a product by 10% and demand rises by 20%, revenue increases (elastic demand).
• Demand shifts ≠ movements along the curve: A drought (supply shock) shifts the supply curve left, raising prices for vegetables in Kathmandu’s New Road market.
• Game theory explains real-world conflicts: Pathao and Uber compete on prices—if one lowers fares, the other must follow (dominant strategy = "match the competitor").

1. Utility Theory: Measuring Consumer Satisfaction

Utility is the satisfaction or happiness a consumer gets from consuming goods/services. Economists classify utility into two approaches:

A. Cardinal Utility (Quantitative)

• Assigns numerical values to satisfaction (e.g., 10 utils for a cup of tea, 20 utils for a movie).
• Limitations:
  • Impossible to measure utils objectively (e.g., how do you compare a burger’s 15 utils to a book’s 10 utils?).
  • Assumes diminishing marginal utility (each additional unit gives less extra satisfaction).

B. Ordinal Utility (Qualitative)

• Ranks preferences without numbers (e.g., "I prefer A > B > C").
• Used in indifference curves and revealed preference theory.
• Real-world use: NEPSE’s stock rankings (e.g., NMB > Global IME) are ordinal.

mindmap
  root((Utility Theory))
    Cardinal
      Numerical values (e.g., 10 utils)
      Diminishing marginal utility
      Limitations: Subjective measurement
    Ordinal
      Rankings (A > B > C)
      Used in indifference curves
      Real-world: NEPSE stock rankings
    Applications
      Consumer choice
      Demand analysis
      Game theory (preference ordering)

2. Budget Constraint and Consumer Choice

Consumers face limited income and must choose between goods. The budget line shows all possible combinations of two goods they can buy with their income.

Key Concepts

• Slope of budget line = Relative price of goods.
  • If Pₓ = Rs. 20, Pᵧ = Rs. 10, and income = Rs. 100, the slope = -2 (steeper than -1).
• Budget line shifts:
  • Income increase → Line shifts right (more of both goods).
  • Price change → Line rotates (e.g., if rice gets cheaper, the line pivots outward on the rice axis).

Worked Example: eSewa’s Data Bundle Choice

Suppose you earn Rs. 2,000/month and can buy:

• Good X: 1 GB data = Rs. 100
• Good Y: 1 hour of internet café = Rs. 50

1. Budget line equation: $100X+50Y=2000$ → $Y=40-2X$
2. If income rises to Rs. 3,000: New budget line: $Y=60-2X$ (shifts right).

pie
  title Consumer's Budget Allocation (Rs. 2,000)
  "Data (1 GB = Rs. 100)" : 20
  "Internet Café (1 hr = Rs. 50)" : 40
  "Remaining" : 40

3. Indifference Curves: Mapping Preferences

An indifference curve shows combinations of two goods that give the same satisfaction.

Properties of Indifference Curves

Worked Example: NTC’s Electricity Plans Suppose a consumer is indifferent between:

• Plan A: 100 units of electricity + 5 kg of firewood.
• Plan B: 80 units of electricity + 6 kg of firewood.

This is one point on their indifference curve. If NTC offers a discount on electricity, the budget line rotates, and the consumer moves to a higher indifference curve.

4. Consumer Equilibrium: Where Budget Meets Preference

Consumer equilibrium occurs where:

1. The budget line is tangent to the indifference curve (highest achievable utility).
2. Slope of budget line = Slope of indifference curve (MRS = price ratio).

Mathematical Condition

$$\frac{M{U}_x}{P_x}=\frac{M{U}_y}{P_y}$$ Where:

• $M{U}_x$ = Marginal utility of good X.
• $P_x$ = Price of good X.

Worked Example: Daraz’s Discounted Products Suppose a consumer buys:

• Smartphone (X): Price = Rs. 20,000, MU = 100 utils.
• Headphones (Y): Price = Rs. 2,000, MU = 20 utils.

For equilibrium: $$\frac{100}{20000}=\frac{20}{2000}⟹0.005=0.01\text{(Not equal!)}$$ → The consumer should buy more headphones (higher MU per rupee).

5. Demand Curve: From Utility to Market Demand

The demand curve shows how much of a good a consumer will buy at different prices, holding other factors constant.

Why Demand is a Flow Concept

• Demand is per unit of time (e.g., "demand for rice per month").
• Unlike stock (total quantity available), demand is dynamic (changes with price/income).

Deriving Demand from Utility

1. Start with marginal utility (MU).
2. As price falls, consumers buy more until MU = Price.
3. Plot these points to get the demand curve.

Worked Example: Kathmandu Traffic Routes Suppose a driver has two routes to work:

• Route A: 30 minutes, MU = 100 (happiness).
• Route B: 20 minutes, MU = 120.

If Route A’s price (time) drops to 20 minutes, the driver switches, shifting demand from Route B to A.

flowchart TD
  A["High Price"] -->|"Consumers buy less"| B["Demand Curve Shifts Left"]
  C["Low Price"] -->|"Consumers buy more"| D["Demand Curve Shifts Right"]
  B --> E["Movement along curve"]
  D --> E

6. Price Elasticity of Demand (PED)

Measures sensitivity of demand to price changes: $$PED=\frac{\%\text{ Change in Quantity Demanded}}{\%\text{ Change in Price}}$$

Types of Elasticity

Worked Example: Ncell’s Data Plans

• Old price: Rs. 500 for 1 GB → Demand = 100 GB.
• New price: Rs. 400 for 1 GB → Demand = 150 GB.

Calculate PED: $$\%\Delta Q=\frac{150-100}{100}\times 100=50\%$$ $$\%\Delta P=\frac{400-500}{500}\times 100=-20\%$$ $$PED=\frac{50}{-20}=-2.5(\text{Elastic})$$ → Ncell should lower prices to increase revenue.

7. Shifts vs. Movements in Demand

Worked Example: Kathmandu’s Flood Impact

• Before flood: Demand for umbrellas = normal.
• After flood announcement: Demand curve shifts right (expectations of rain).

8. Game Theory: Strategic Consumer Choices

Game theory studies interdependent decisions (e.g., competitors, negotiations).

Dominant Strategy

A strategy that is best regardless of what others do.

Example: Pathao vs. Uber Pricing

• Dominant strategy for both: Low price (maximizes minimum payoff).
• Nash Equilibrium: Both choose low price (Rs. 50k each).

Real-world tie-in: Nepal’s eSewa vs. Khalti fee wars—both lowered transaction costs to attract users.

In the Real World

1. eSewa’s Dynamic Pricing

   • Uses price elasticity to adjust transaction fees. If demand for online payments rises (e.g., during Dashain), eSewa may increase fees slightly (inelastic demand for essential payments).
   • Utility theory: Consumers rank convenience (eSewa) > cash, so indifference curves slope toward digital payments.

2. Daraz’s Discount Strategies

   • During sales, Daraz lowers prices on electronics (elastic demand) but keeps essentials (like rice) stable (inelastic).
   • Budget constraint: A Rs. 30,000/month salary limits choices—discounts shift the budget line outward.

3. NTC’s Load Shedding Announcements

   • When NTC announces longer load shedding, demand for solar panels and generators shifts right (expectations).
   • Indifference curves: Consumers trade off between electricity (X) and backup power (Y), moving to higher utility curves.

Exam Tip

1. Diagrams are worth 50% of marks:

   • Always draw budget lines, indifference curves, and demand/supply curves with labels.
   • For PED, show elastic vs. inelastic segments and mark revenue changes.

2. Numerical questions:

   • For PED, always use midpoint formula (avoids sign errors): $$PED=\frac{(Q_2-Q_1)/(Q_2+Q_1)}{(P_2-P_1)/(P_2+P_1)}$$
   • For equilibrium, set MU = Price and solve.

3. Case studies:

   • Link to Nepal’s context (e.g., "If NTC increases electricity tariffs by 20%, how will demand change?").
   • Use real data: Nepal’s inflation rate (10% in 2023) affects demand shifts.

4. Game theory:

   • Always identify payoffs and circle the dominant strategy.
   • Example: "If two duopolists (e.g., NMB and Global IME) collude, what’s the Nash equilibrium?"

5. Common mistakes to avoid:

   • Confusing movements along a curve (price change) with shifts (non-price factors).
   • Forgetting diminishing marginal utility in indifference curves (they must be convex).
   • Ignoring units in elasticity calculations (e.g., % change, not absolute change).

Final Visual Summary

classDiagram
  class Consumer {
    +Income: Rs.
    +Preferences: Ordinal/Cardinal
    +Budget Line: Y = a - bX
    +Equilibrium: MUx/Px = MUy/Py
  }
  class Market {
    +Demand Curve: Qd = a - bP
    +Elasticity: PED > 1 (Elastic)
    +Shifts: Income, Tastes, Expectations
  }
  class GameTheory {
    +Players: Firms/Consumers
    +Strategies: Price, Quantity
    +Outcome: Nash Equilibrium
  }
  Consumer --> Market : "Chooses Qd"
  Market --> GameTheory : "Strategic Interaction"