Track D: Chapter 21 — Scenarios + sensitivity + stress testing

Makefile

@@ -412,6 +412,10 @@ business-ch19:
 business-ch20:
 	$(PYTHON) -m scripts.business_ch20_integrated_forecasting_three_statements --datadir $(OUT_NSO_V1) --outdir $(OUT_TRACK_D) --seed $(SEED)
 
+.PHONY: business-ch21
+business-ch21:
+	$(PYTHON) -m scripts.business_ch21_scenario_planning_sensitivity_stress --datadir $(OUT_NSO_V1) --outdir $(OUT_TRACK_D) --seed $(SEED)
+
 
 # --- Quality gates ---
 .PHONY: lint

docs/source/business_ch21_scenario_planning_sensitivity_stress.rst

@@ -0,0 +1,91 @@
+Business Statistics & Forecasting for Accountants (Track D)
+===========================================================
+
+Chapter 21 — Scenario planning, sensitivity analysis, and stress testing
+------------------------------------------------------------------------
+
+This chapter turns your integrated forecast into a **decision tool**:
+
+* **Scenarios**: best/base/worst forecasts tied to explicit driver assumptions.
+* **Sensitivity**: a simple “what matters most?” scan to find the levers that move cash risk.
+* **Stress tests**: conservative shocks that answer: *What if things go wrong at the same time?*
+
+The focus is accountant-friendly:
+
+* interpret coefficients as **rates and baselines**,
+* avoid causal overclaiming,
+* use guardrails (cash buffer triggers, documentation, and governance).
+
+Learning objectives
+^^^^^^^^^^^^^^^^^^^
+
+By the end of Chapter 21 you should be able to:
+
+* Build a **best/base/worst** scenario pack tied to driver assumptions.
+* Run a **sensitivity scan** to identify the top levers affecting cash shortfall risk.
+* Create a **stress test narrative** suitable for a lender or board packet.
+* Communicate downside risk conservatively with a clear audit trail.
+
+Direct, practical idea
+^^^^^^^^^^^^^^^^^^^^^^
+
+Scenario planning works best when you keep the model simple:
+
+* Revenue assumptions (level / multiplier)
+* Gross margin assumptions (COGS as a rate of revenue)
+* Operating expense assumptions (Opex as a rate of revenue)
+* Working capital behavior (DSO / DIO / DPO as “days”)
+* Cash policy (buffer targets and trigger thresholds)
+
+The script for this chapter uses the NSO v1 simulator tables and produces a deterministic
+scenario pack plus a one-at-a-time sensitivity summary.
+
+How to run
+^^^^^^^^^^
+
+From the repo root:
+
+.. code-block:: bash
+
+   # (Re)generate NSO v1 synthetic data (if needed)
+   make business-nso-sim
+
+   # Run Chapter 21
+   make business-ch21
+
+Outputs
+^^^^^^^
+
+Artifacts are written under ``outputs/track_d/``:
+
+* ``ch21_scenario_pack_monthly.csv`` — 12-month scenario pack (Base / Best / Worst / Stress)
+* ``ch21_sensitivity_summary.csv`` — one-at-a-time sensitivity scan (cash buffer risk)
+* ``ch21_assumptions.csv`` — scenario assumptions log (explicit, auditable)
+* ``ch21_cash_governance_template.csv`` — governance checklist (who/when/trigger/action)
+* ``ch21_design.json`` — reproducibility metadata
+* ``ch21_memo.md`` — short interpretation memo
+* ``ch21_figures_manifest.csv`` + ``figures/ch21_fig_*.png`` — figure list + plots
+
+Interpretation guardrails
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+* Treat scenario results as **planning ranges**, not forecasts you can “prove.”
+* If a scenario shows cash risk, the right response is **controls + mitigation options**:
+
+  * tighten collections (DSO)
+  * adjust purchasing/inventory (DIO)
+  * renegotiate payment terms (DPO)
+  * delay non-critical capex
+  * change owner draws / distributions
+
+* Do not claim that a lever *causes* a cash change unless you can justify the mechanism.
+
+End-of-chapter problems
+^^^^^^^^^^^^^^^^^^^^^^^
+
+1. **Three-scenario pack**: Create best/base/worst assumptions and summarize differences
+   in one page (cash buffer triggers and recommended mitigations).
+2. **Sensitivity**: Identify the top 3 levers impacting cash shortfall risk and explain
+   why each lever matters operationally.
+3. **Stress test memo**: Draft a lender-ready explanation of your stress test logic:
+   assumptions, conservatism, and action plan.

docs/source/index.rst

@@ -170,5 +170,6 @@ See :doc:`getting_started` and :doc:`contributing` for the contributor workflow.
    business_ch18_expense_forecasting_fixed_variable_step_payroll
    business_ch19_cash_flow_forecasting_direct_method_13_week
    business_ch20_integrated_forecasting_three_statements
+   business_ch21_scenario_planning_sensitivity_stress
    business_appendix_pdf_refresher
    business_appendix_authoring_rules