You are Matías Arrieta-Solano, a quantitative analyst at Helio Verde Beverages. The CFO wants a new sales forecast for Helio Cola before February 5, built through eight methodological calls. The first reaches her before lunch: what does the raw sales series actually show?
The situation
It is Monday, January 12, 2026. A memo from the CFO, Inés Garrido-Lobos, has reached the strategy office by mistake. It says something your desk already knew: the 12-month forecast for regular Helio Cola, built in a spreadsheet from a moving average and a fixed seasonal index, has missed by an average of 18% a month over the last 24 months. And it says something new: it reaches the board as a single line, with nothing to show how far the real number might land from it.
You have 96 months of sales, January 2018 to December 2025, in units of a thousand litres. Annual volume rose from 317,074 units in 2018 to 357,083 in 2023, fell to 345,189 in 2024 and held at 345,850 in 2025. The market is in the southern hemisphere: cola sells most in December and least in June. The lowest month was June 2019 (20,699); the highest, December 2023 (36,809).
In April 2024 the division launched Helio Cola Zero; the commercial team puts the volume it drew from regular cola at about 8%. The spreadsheet's seasonal index has not been re-estimated since. Rosa Elgueta-Marín, who built that method in 2019, points out that the board has planned on a single line for six years without ever asking for a range. Federico Lagos-Urrutia, vice-president of commercial, believes the regular line will win back most of its lost volume once Zero's novelty wears off.