Three-Statement Modeling Mechanics and Integrity Checks
Financial Modeling
A three-statement model forecasts the income statement, balance sheet and cash flow statement so that they tie together through retained earnings, debt and working capital. Build in this order: (1) operating assumptions and drivers, (2) income statement, (3) balance sheet schedules (PP&E roll-forward, debt schedule, working capital, equity), (4) cash flow statement, (5) checks.
The key mechanical issue is circularity: interest expense depends on the debt balance, which depends on cash flow, which depends on interest expense. Models solve this with a copy-paste macro, by enabling iterative calculation, or - cleanest and most common - by computing interest on the average of opening and closing balances.
Traps that show up in questions: sign conventions (outflows negative on the cash flow statement but positive on the model's capex line), hardcoded numbers buried inside formulas, mixed units (thousands vs millions), forecast periods that do not roll forward, and a balance sheet that plugs to cash rather than to a check cell. Never let a model balance by adding a plug; a plug simply hides the error.
Every model needs a checks page: assets minus liabilities minus equity equals zero, the cash flow statement ties to the change in cash, sources equal uses in any transaction section, and debt never goes negative.
Worked rule of thumb: if debt is 1,000 at the start of the year and 1,200 at the end with 8% interest, interest expense is about 8% x 1,100 = 88, not 80 or 96. Average balances remove the circularity and stay within a few percent of the iterated answer.