PyPI workbook run (Track D)

From inside your Track D workbook folder (created by pystatsv1 workbook init --track d --dest ...), run:

pystatsv1 workbook run |trackd_run|

Outputs are written under outputs/track_d/ by default. If you’re unsure what a file is for, start with Track D Outputs Guide.

To see the full chapter-by-chapter run map (D00–D23), see Track D chapter index (PyPI).

Optional: write to a custom output folder:

pystatsv1 workbook run |trackd_run| --outdir outputs/track_d_custom

Interpretation prompts (quick self-check):

What is the accounting or business measurement goal in this chapter?
Which invariant/check would catch a “numbers look fine but are wrong” mistake here?

Track D — Chapter 18

Expense forecasting (fixed / variable / step costs, with payroll scenarios)

Revenue forecasts are only half of planning. For most small businesses, expense behavior is what turns a “forecast” into an action plan.

This chapter introduces an accountant-friendly way to forecast expenses without pretending you have a perfect cost model:

Fixed costs: largely unchanged month to month (rent, many subscriptions).
Variable costs: move with activity (utilities, processing fees, shipping).
Step costs: stay flat until you cross a capacity threshold (staffing / payroll).

Payroll is the most important step cost in the NSO running case, so we build a simple payroll forecast using scenarios.

What you will build

1) An expense table by account (monthly)

You will compute monthly totals for key operating expense accounts from the GL:

Rent (6100)
Utilities (6200)
Payroll (6300)
Depreciation (6400)
Payroll tax (6500)
Interest (6600)

COGS is excluded here on purpose: COGS is usually modeled alongside revenue.

2) A cost behavior map

You will produce a mapping that classifies each expense line by:

cost behavior (fixed / variable / step)
controllable vs non-controllable
suggested forecasting method
suggested monitoring KPI

This is the bridge between “statistics” and “budget accountability.”

3) Payroll forecast with scenarios

Payroll is forecast using a simple driver model:

start with a baseline monthly gross wages estimate from history
apply a scenario multiplier (Lean / Base / Growth)
apply a small monthly wage inflation assumption
estimate employer payroll tax using the historical rate (from the simulator)

This is not a causal model. It is a planning model.

Data source

This chapter uses:

gl_journal.csv + chart_of_accounts.csv (expense totals by month)
payroll_events.csv (payroll accruals and employer tax accruals)

How to run Chapter 18

Prerequisite: generate the NSO dataset (once)

If you already ran Chapters 14–17, you already have NSO v1 in:

data/synthetic/nso_v1

If not:

make business-nso-sim
make business-validate

Run the Chapter 18 analysis

make business-ch18

By default this runs:

python -m scripts.business_ch18_expense_forecasting_fixed_variable_step_payroll \
  --datadir data/synthetic/nso_v1 \
  --outdir outputs/track_d \
  --seed 123

Outputs

All artifacts are written under:

outputs/track_d

Core tables (CSV)

ch18_expense_monthly_by_account.csv History table: one row per month, one column per key expense account.
ch18_expense_behavior_map.csv The cost behavior + controllability map (this is a “budget rubric”).
ch18_payroll_monthly.csv Payroll accrual history by month (gross wages + employer tax).
ch18_payroll_scenarios_forecast.csv Payroll forecast for the next 12 months under Lean/Base/Growth scenarios.
ch18_expense_forecast_next12_detail.csv Long-form forecast table (month × scenario × account line).
ch18_expense_forecast_next12_summary.csv Wide summary (month × scenario) with totals and controllable subtotal.
ch18_control_plan_template.csv A starter “expense control plan” table (owners / KPIs / cadence).

Design + narrative (JSON/MD)

ch18_design.json Machine-readable assumptions (scenario multipliers, inflation, employer tax rate).
ch18_memo.md A short planning memo with the next-12 summary table.

Figures (PNG) + manifest

Figures are written under:

outputs/track_d/figures

and listed in:

ch18_figures_manifest.csv

Figures include:

expense history by category
payroll forecast overlay (Lean/Base/Growth)

Interpretation guardrails

Treat the outputs as a budget baseline, not “the truth.”
Do not claim that revenue causes expenses in this chapter. We only classify and project.
Always pair the forecast with an assumptions log and owners for the controllable lines.

End-of-chapter exercises

Add a new scenario (e.g., “Aggressive growth”) and explain the staffing plan behind it.
Replace the utilities forecast with a simple “% of revenue” assumption. Compare results.
Build a one-page expense control dashboard spec: thresholds + owners + escalation rules.