ARIMA Said “We’re Growing.” Finance Said “We Missed.” Now What?

If your time-series forecast looks strong but the budget still gets missed, that is the disconnect.

Author

Glenda Carnate
February 27, 2026

Read time: 2.5 minutes

The data team confidently reports: "The ARIMA forecast is on an upward trend" with seasonality appropriately represented and all standard error metrics are acceptable and charts are pointing north.

Then the quarter ends.

Finance asks a single question: "How did we miss the budget?"

Because the model forecasted history would continue, while the budget assumed ambition would drive acceleration.

Why aren't forecasting & budgeting the same?

  1. ARIMA provides a historical perspective.
    It assumes the end of a trend will lead to the beginning of another, but does not account for new growth opportunities.

  2. Budgeting represents a company's expectations.
    Your expected growth is based on the assumption that you will continue to grow and/or improve your performance.

  3. A trend can be on the rise, but it will not necessarily catch up to the original forecast.
    Revenues can continue to grow while still being below the overall revenue growth target.

  4. Forecasting answers the question of "What is most likely to happen?"; budgeting answers the question of "What must happen?"

  5. The real issue with gap analysis is that if you are not specifically evaluating your forecast against the actuals, you are comparing two different sets of numbers.  
    Statistically, the model can still be forecasted correctly, but it may not be in a strategic manner.

💡Key Takeaway: 

While both are based on predictions, the purpose of budgeting is to quantify and measure total performance against expected outcomes.

Ignoring this relationship will lead to continuous meetings discussing the same issues.

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