sensitivity-analysis - SKILL.md Agent Skill

name: sensitivity-analysis description: Systematic testing of how changes in input variables (assumptions) affect output results, revealing which factors drive outcomes and where model risk is concentrated.

Sensitivity Analysis

Canonical Source: Financial Modeling / Operations Research Domain: Finance, Strategy, Engineering, Decision Analysis Common Tools: Excel Data Tables, Tornado Charts, What-If Analysis

One-Line Summary

Systematic testing of how changes in input variables (assumptions) affect output results, revealing which factors drive outcomes and where model risk is concentrated.

Core Concept

Sensitivity Analysis answers: "What matters most?" and "How wrong can I be?" It stress-tests assumptions by varying inputs one-at-a-time (or in combination) to measure impact on key outputs.

The Core Technique: Change one variable while holding others constant → measure output change → repeat for all variables → rank by impact.

The Innovation: Transforms opaque models ("trust the spreadsheet") into transparent risk maps ("here's what we're betting on").

When to Use

Ideal Scenarios:

Financial modeling (DCF valuation, project finance, pricing models)
Strategic planning with uncertain market assumptions
Engineering design trade-offs (cost vs. performance)
Risk assessment to identify critical vulnerabilities
Communicating model assumptions to non-technical stakeholders
Validating that models behave logically (sanity checks)

Not Suitable For:

When you need to test many variables simultaneously (use Monte Carlo instead)
Highly non-linear systems where one-at-a-time testing misses interactions
When precise probability distributions matter more than ranges
Decisions requiring deep causal understanding (sensitivity shows correlation, not causation)

Execution Steps

1. Define Base Case Model

Build a model with clear inputs → calculations → outputs
Set baseline assumptions for all variables (most likely values)
Identify the KEY OUTPUT metric (NPV, IRR, profit, cost, etc.)
Validate base case logic and formulas

Output: Working model with base case result

2. Identify Input Variables to Test

List all assumptions that drive the output
Focus on variables with genuine uncertainty (not fixed constraints)
Typical candidates: growth rates, costs, prices, volumes, discount rates
Limit to 5-10 most critical inputs for initial analysis

Output: Ranked list of input variables

3. Define Testing Ranges

For each input, set a plausible range (low/high bounds)
Use historical volatility, expert estimates, or ±10-20% heuristic
Ensure ranges are realistic (not theoretically possible but practically absurd)
Document reasoning for each range

Output: Range table (e.g., Revenue Growth: 5%-15%, Base: 10%)

4. Run One-Variable Sensitivity (One-Way)

Change ONE input from low → high while keeping others at base
Record output at each step (often 5-7 data points across range)
Repeat for each input variable independently
Calculate output swing: (Max Output - Min Output) / Base Output

Output: Sensitivity table showing output for each input variation

5. Visualize Results

Tornado Chart: Horizontal bar chart ranking variables by output impact (widest = most sensitive)
Sensitivity Table: Matrix showing output values across input ranges
Spider Chart: Lines radiating from base case showing output response to each input

Output: Visual representation highlighting key drivers

6. Run Two-Variable Sensitivity (Two-Way)

Select the 2-3 most impactful variables from Step 4
Create a matrix: vary Variable A (rows) and Variable B (columns) simultaneously
Excel Data Table is perfect for this (rows = input 1, columns = input 2, cells = output)
Visualize with heat maps or contour plots

Output: 2D sensitivity matrix (e.g., NPV across revenue growth × cost inflation combinations)

7. Interpret and Communicate

Identify HIGH-IMPACT inputs: Small changes → big output swings (focus risk mitigation here)
Identify LOW-IMPACT inputs: Wide ranges → minor output change (deprioritize)
Define "safe zones" vs. "danger zones" in two-way tables
Communicate: "Our model is most sensitive to X; if X falls below Y, project fails"

Output: Executive summary with risk priorities and decision thresholds

Common Pitfalls

"One-at-a-Time" Blindness Testing variables individually misses interaction effects. Revenue growth + high costs might be survivable, but testing them separately hides the combined threat.

Solution: Follow one-way analysis with two-way (or three-way) for critical variable pairs. Use Monte Carlo for complex interactions.

Arbitrary Ranges Setting input ranges like ±50% just because it's a round number, ignoring actual historical volatility.

Solution: Ground ranges in data (standard deviations, historical min/max, expert calibrated estimates).

Confusing Sensitivity with Probability High sensitivity means "this variable matters IF it changes," not "this variable WILL change significantly."

Solution: Combine sensitivity analysis with probability assessments (which variables are both high-impact AND likely to vary?).

Analysis Paralysis Testing 30 variables and overwhelming stakeholders with information.

Solution: Focus on the vital few (80/20 rule). Start with 5-7 key drivers, expand only if needed.

Key Insights

Tornado Charts Rule: Named for their shape (widest bars at top, narrowing down), tornado charts instantly communicate "what matters" to executives. They're the gold standard for sensitivity visualization.

Excel's Hidden Power: Data Tables (What-If Analysis tool) automate sensitivity calculations. One-variable tables test a single input across a range; two-variable tables create matrices testing two inputs simultaneously.

Model Validation Trick: If changing a variable produces counterintuitive results (higher price → lower revenue in isolation), you've found a model bug or faulty logic.

The "Swing Weight": In decision analysis, swing weight = the range of output values as an input swings from low to high. Larger swing = higher priority for risk management or better information gathering.

Real-World Application

DCF Valuation: Valuing a startup with huge uncertainty. Sensitivity analysis tests revenue growth (5-25%), operating margin (10-30%), and discount rate (12-20%). Tornado chart reveals discount rate drives 60% of valuation variance → negotiate equity terms around cost of capital.

Project Finance: Solar farm NPV model depends on electricity price, construction cost, and tax incentives. Two-way table (price × cost) shows project is viable only if price stays above $0.08/kWh AND costs below $1.2M/MW. This defines the "go/no-go" zone for final investment decision.

Pricing Strategy: SaaS company tests pricing model sensitivity to churn rate (2-8%), conversion rate (10-25%), and CAC (50-200). Analysis shows churn has 3x impact of conversion → prioritize retention over acquisition.

Related Frameworks

Tornado Diagram: Visual output of sensitivity analysis, ranking variables by impact
Monte Carlo Simulation: Probabilistic version (tests all variables simultaneously with distributions)
Scenario Analysis: Tests discrete scenarios (best/base/worst) vs. continuous ranges
Break-Even Analysis: Special case finding the input value where output = 0
Value at Risk (VaR): Sensitivity analysis applied to portfolio loss distributions
Stress Testing: Extreme sensitivity analysis (push inputs to crisis levels)

Anti-Patterns

"It's All Red" Syndrome Setting every variable to worst-case simultaneously to show "look how risky this is." That's not sensitivity analysis—it's fear-mongering.

Output Overload Generating 50-page sensitivity reports with every possible variable combination, ensuring nobody reads it.

Ignoring Correlations Testing oil price sensitivity and shipping cost sensitivity independently when they're highly correlated (both driven by crude prices).

"Set It and Forget It" Running sensitivity analysis once during planning and never updating as assumptions change or new data emerges.

Score Justification

Framework Assessment: 44/50 (Tier 1 - Canonical)

Practitioner Weight (9/10): Ubiquitous in investment banking, corporate finance, consulting (McKinsey/Bain/BCG all use religiously), and engineering. Every financial model includes sensitivity analysis. Minor deduction: sometimes performed mechanically without insight.
Clarity & Executability (9/10): Extremely clear concept. Excel makes execution trivial (Data Tables automate calculations). Anyone can run a tornado chart within an hour of learning.
Proven ROI (9/10): Prevents disasters by revealing fragile assumptions. McKinsey studies show companies using sensitivity analysis outperform in volatile markets. Saved countless projects from over-optimistic projections.
Novelty (7/10): Conceptually straightforward (vary inputs, measure outputs). The insight is systematic application and visualization (tornado charts), not mathematical innovation.
Cross-Domain Applicability (10/10): Universal. Finance, engineering, pharma R&D, public policy, climate modeling, supply chain optimization, marketing mix modeling—any domain with uncertain inputs.

Notable: Excel's Data Table feature (introduced 1990s) democratized sensitivity analysis. Before spreadsheets, this was tedious manual calculation. Now it's 3 clicks. Tornado charts became the de facto standard for communicating model risk to boards and executives.