Shock Elasticities
Work note
Overview
In economic modeling, short-term elasticity differs significantly from steady-state figures. A “shock”—whether a sudden price spike or a recessionary income drop—forces a transition from habitual behavior to active adaptation. This document summarizes how to conceptualize these shifts and the risks associated with adjusting elasticity coefficients during uncertainty.
1. The Mechanics of a Short-Term Shock
During the immediate onset of a shock, demand is typically less elastic than in the long run. This is driven by three primary factors:
- Behavioral Inertia: Habits and existing contracts prevent immediate changes in consumption.
- Infrastructure Constraints: Physical limitations (e.g., owning a specific car or living a certain distance from work) lock consumers into spending patterns.
- The Adjustment Lag: It takes time for consumers to identify substitutes or change lifestyle patterns.
2. Income Elasticity (YED) Benchmarks
The impact of a recession is measured by how sensitive a category is to changes in consumer income.
| Good Type | Steady-State YED | Response to Income Shock |
|---|---|---|
| Necessities | 0.0 to 0.5 | Spending is protected; cuts are minimal. |
| Normal Goods | 0.5 to 1.5 | Spending fluctuates in tandem with the economy. |
| Luxuries/Durables | > 1.5 | Spending is highly volatile; often the first to be cut. |
| Inferior Goods | < 0.0 | Demand increases as consumers “trade down.” |
3. Analysis of the “Unitary Elasticity” (1.0) Assumption
You proposed shifting a category from a steady-state 0.2 to a shock-state 1.0.
The Rationale
- Neutrality: When specific data is unavailable, 1.0 serves as a “societal average” or a mathematical middle ground.
- The Principle of Insufficient Reason: If there is no evidence to suggest a specific direction, assuming the category mirrors the overall economy is a defensible placeholder.
The Risks
Warning: Moving from 0.2 to 1.0 is a 5x increase in sensitivity.
- Over-Correction: If a category is fundamentally a necessity (0.2), it is unlikely to behave like a normal good (1.0) even in a shock. This may lead to an overly pessimistic revenue forecast.
- Identity Shift: Setting YED to 1.0 assumes the product’s “budget share” remains constant, ignoring the fact that consumers usually prioritize low-elasticity goods when budgets tighten.
The category has low elasticity, but is not a necessity, so the objection does not hold fully.
4. Strategic Recommendations
To improve model robustness, consider the following:
- Sensitivity Analysis: Rather than a single point (1.0), run scenarios at 0.4, 0.7, and 1.0.
- Sector Benchmarking: Use the average elasticity of the specific sector (e.g., CPG, Tech, or Utilities) rather than the total societal average.
- Time-Phased Modeling: Apply a higher elasticity for the peak of the shock, but decay it back toward the steady-state 0.2 as “new normals” are established.
Key Takeaway: While 1.0 is a convenient “best guess,” it fundamentally changes the nature of the product from a necessity to a discretionary item. Ensure your strategy can withstand the volatility this assumption introduces. “””